Include line numbers in backtraces

[gsingh93]

When a binary compiled with -g panics, and RUST_BACKTRACE=1, line numbers should be given along with the backtrace.

[brooksbp]

It looks like backtrace_pcinfo() could be used instead of _Unwind_FindEnclosingFunction() + backtrace_syminfo(). I'll attempt it.

[lifthrasiir]

I have an almost working version of this and am working on a PR. Some caveats I've discovered:

• It will only work in non-Android POSIX as it depends on libgcc and libbacktrace.
• We really need _Unwind_GetIPInfo instead of _Unwind_GetIP to determine if given IP is a non-signaling frame (thus the IP is located after the calling instruction). GCC 4.2.0 or later has this, so it should be fine, but we need a workaround for non-libgcc paths.
• backtrace_pcinfo can be called multiple times for inlined calls. For the convenience I have 32 calls limit right now.
• Macros somehow replaces the file/line information for given frame, which makes a test quite annoying. panic!() for example will replace the last user frame with <std macros>:3 instead of a line invoking the macro. Really an issue at its own.

[brooksbp]

I was having difficultly ramping up on Rust's FFI and did not make much progress on this. @lifthrasiir Looking forward to your work.

[kornelski]

Oops, I've filed a duplicate #24346, because it doesn't work for me in rustc 1.0.0-nightly (6436e34 2015-04-08) (built 2015-04-08)