Debuginfo revamp
Disable struct value printing in debug info tests

This is due to the way different versions of gdb print out structs: older versions have them always spread out with fields on different lines, while newer versions will compactly print them on one line. This makes it hard for the output checker to verify the expected output.
Re-enable struct value printing in debug info tests with gdb pretty printing off
More descriptive error messages for debug info tests
Avoid propagating link_arg values that are unlikely to be resolveable under arbitrary directories.
Remove uses of log
core: Remove logging constants
Remove the log keyword (by renaming it to __log)

We can't quite remove logging from the language, but this hides the
keyword.
auto merge of #5332 : jdm/rust/transitivelink, r=graydon

The original change bit Servo because rust-harfbuzz includes libharfbuzz.a in its link_args. This works fine in the rust-harfbuzz subdirectory where the static library resides, but when this is propagated to servo_gfx, the lirbrary can no longer be found since it's a relative path.
Disable debuginfo tests on mac since gdb requires root
core: Turn off rtdebug logging
libcore: Attempt to put out burning tree on Mac by using the old symbol on 32 bit. rs=bustage
Work around linkage bug cross-compiling from x86_64-apple-darwin to i686-apple-darwin

The correct opendir/readdir to use appear to be the 64-bit versions called
opendir$INODE64, etc. but for some reason I can't get them to link properly
on i686. Putting them in librustrt and making gcc figure it out works.
This mystery will have to wait for another day.
auto merge of #5293 : brson/rust/logging, r=brson

r? @graydon

This removes `log` from the language. Because we can't quite implement it as a syntax extension (probably need globals at the least) it simply renames the keyword to `__log` and hides it behind macros.

After this the only way to log is with `debug!`, `info!`, etc. I figure that if there is demand for `log!` we can add it back later.

I am not sure that we ever agreed on this course of action, though I *think* there is consensus that `log` shouldn't be a statement.
auto merge of #5319 : brson/rust/debuginfo, r=brson

Continuing #5140 

For the sake of getting this merged I've disabled debuginfo tests on mac (where running gdb needs root). Please feel free to follow up with further improvements.