Define rustc's host triple at compile time

This way a cross-compiled rustc's answer to host_triple() is correct. The return
value of host_triple() reflects the actual host triple that the compiler was
build for, not the triple the compiler is being built on
Un-ignore rusti/rustpkg tests on i686
Replaced calls to external fmin/fmax by a Rust implementation.
Changed to a more efficient implementation.
auto merge of #6733 : alexcrichton/rust/issue-2400, r=brson

Most of the relevant information can be found in the commit messages.

r? @brson - I just wanted to make sure the make changes aren't completely bogus

This would close #2400, #6517, and #6489 (although a run through incoming-full on linux would have to confirm the latter two)
auto merge of #6803 : Thiez/rust/native_fmax_fmin, r=brson

Calls to the libc versions of fmin and fmax were relatively slow (perhaps because they could not be inlined?). This pull request provides f32 and f64 with fmin and fmax written in Rust, and shows a significant speed increase on my system; I used https://github.com/thiez/rustray as my benchmark, with --opt-level 3 it brings the ray-tracing time down from 10.8 seconds to about 9.2, which seemed significant to me.

r?
rustc: Remove unused import
rustc: Improve astconv error message
syntax: Add an each_view_item method on traits
rustc: Call str::is_empty
rustc/rusti/rustpkg: Infer packages from `extern mod` directives

This commit won't be quite as useful until I implement RUST_PATH and
until we change `extern mod` to take a general string instead of
an identifier (#5682 and #6407).

With that said, now if you're using rustpkg and a program contains:

extern mod foo;

rustpkg will attempt to search for `foo`, so that you don't have to
provide a -L directory explicitly. In addition, rustpkg will
actually try to build and install `foo`, unless it's already
installed (specifically, I tested that `extern mod extra;` would
not cause it to try to find source for `extra` and compile it
again).

This is as per #5681.

Incidentally, I changed some driver code to infer the link name
from the crate link_meta attributes. If that change isn't ok, say
something. Also, I changed the addl_lib_search_paths field in the
session options to be an @mut ~[Path] so that it can be modified
after expansion but before later phases.
testsuite: Add an xfailed test case for the new `extern mod` syntax