log: Introduce liblog, the old std::logging

This commit moves all logging out of the standard library into an external
crate. This crate is the new crate which is responsible for all logging macros
and logging implementation. A few reasons for this change are:

* The crate map has always been a bit of a code smell among rust programs. It
  has difficulty being loaded on almost all platforms, and it's used almost
  exclusively for logging and only logging. Removing the crate map is one of the
  end goals of this movement.

* The compiler has a fair bit of special support for logging. It has the
  __log_level() expression as well as generating a global word per module
  specifying the log level. This is unfairly favoring the built-in logging
  system, and is much better done purely in libraries instead of the compiler
  itself.

* Initialization of logging is much easier to do if there is no reliance on a
  magical crate map being available to set module log levels.

* If the logging library can be written outside of the standard library, there's
  no reason that it shouldn't be. It's likely that we're not going to build the
  highest quality logging library of all time, so third-party libraries should
  be able to provide just as high-quality logging systems as the default one
  provided in the rust distribution.

With a migration such as this, the change does not come for free. There are some
subtle changes in the behavior of liblog vs the previous logging macros:

* The core change of this migration is that there is no longer a physical
  log-level per module. This concept is still emulated (it is quite useful), but
  there is now only a global log level, not a local one. This global log level
  is a reflection of the maximum of all log levels specified. The previously
  generated logging code looked like:

    if specified_level <= __module_log_level() {
        println!(...)
    }

  The newly generated code looks like:

    if specified_level <= ::log::LOG_LEVEL {
        if ::log::module_enabled(module_path!()) {
            println!(...)
        }
    }

  Notably, the first layer of checking is still intended to be "super fast" in
  that it's just a load of a global word and a compare. The second layer of
  checking is executed to determine if the current module does indeed have
  logging turned on.

  This means that if any module has a debug log level turned on, all modules
  with debug log levels get a little bit slower (they all do more expensive
  dynamic checks to determine if they're turned on or not).

  Semantically, this migration brings no change in this respect, but
  runtime-wise, this will have a perf impact on some code.

* A `RUST_LOG=::help` directive will no longer print out a list of all modules
  that can be logged. This is because the crate map will no longer specify the
  log levels of all modules, so the list of modules is not known. Additionally,
  warnings can no longer be provided if a malformed logging directive was
  supplied.

The new "hello world" for logging looks like:

    #[phase(syntax, link)]
    extern crate log;

    fn main() {
        debug!("Hello, world!");
    }
rustc: Remove compiler support for __log_level()

This commit removes all internal support for the previously used __log_level()
expression. The logging subsystem was previously modified to not rely on this
magical expression. This also removes the only other function to use the
module_data map in trans, decl_gc_metadata. It appears that this is an ancient
function from a GC only used long ago.

This does not remove the crate map entirely, as libgreen still uses it to hook
in to the event loop provided by libgreen.
rustc: Tweak where -lmorestack is on link commands

In removing many fields from the crate map, executables no longer always have an
explicit dependency on all upstream libraries. This means that the linker is no
longer picking them up as it used to.

To the best of my knowledge, the current situation is happening:

* On linux, we're passing the --as-needed flag to the linker, meaning that
  libraries are stripped out if there are no references to symbols in them.
* Executables may not reference libstd at all, such as "fn main() {}"
* When linking, the linker will discard libstd because there are no references
  to symbols in it. I presume that this means that all previous libs have had
  all their symbols resolved, so none of the libs are pulling in libstd as a
  dependency.
* The only real dependence on libstd comes from the rust_stack_exhausted symbol
  (which comes from libmorestack), but -lmorestack is at the end so by the time
  this comes up libstd is completely gone, leading to undefined references to
  rust_stack_exhausted

I'm not entirely convinced that this is what's happening, but it appears to be
along these lines. The one thing that I'm sure of is that removing the crate map
(and hence implicit dependency on all upstream libraries) has changed how
objects depend on upstream libraries.
Test fixes and rebase conflicts

This commit switches over the backtrace infrastructure from piggy-backing off
the RUST_LOG environment variable to using the RUST_BACKTRACE environment
variable (logging is now disabled in libstd).
auto merge of #12791 : alexcrichton/rust/liblog, r=brson

The rationale and modifications can be found in the first commit message.

This does make logging a bit more painful to use initially because it involves a feature gate and some `phase` attributes, but I think it may be reasonable to not require the `phase` attribute for loading `macro_rules!` macros because defining them will still be gated.