Removing the no-omit-frame-pointer LLVM option breaks Mac stack traces

[catamorphism]

I was getting gdb backtraces (on Mac OS X) that were only two frames long, when they shouldn't have been. @brson suggested that I try reverting 0ac02e7 , @thestinger 's commit that eliminates the no-omit-frame-pointer option. When I reverted everything including and after this commit, I was able to get correct backtraces again.

I don't understand this area well enough to say more, but @brson thought that if this was breaking Mac stack traces, the patch should be reverted for now.

[thestinger]

gdb will fall back to using frame pointers if the regular call frame info is unavailable, so this makes sense. I'm not sure why the CFI stuff wouldn't be working on OS X. Note that lldb on OS X is totally broken with Rust too: #8543, but it doesn't do the frame pointer fallback.