Demangle symbols in assembly output

[Tuj3Bz90dV]

I believe symbols in Rust are mangled using the C++ mangler format, to make things easier on gdb. By piping the assembly output (i.e., the result of running rustc --emit asm -o -) to the standard c++filt utility, one can get

callq   rand::TaskRng.Rng::next_u64::hb4b8897f3e175566FFb::v0.11.0.pre@PLT

instead of the current

callq   _ZN4rand11TaskRng.Rng8next_u6420hb4b8897f3e175566FFb11v0.11.0.preE@PLT

I find that this helps readability when looking at assembly dumps.

[thestinger]

I think this would be a good idea, but it would probably be best as a setting.