mk: remove -fno-omit-frame-pointer

This is still used for Rust code (`Options.NoFramePointerElim = true`).
stop disabling frame pointer elimination for Rust

This no longer breaks debugging on OS X as verified by @alexcrichton.

Closes #7477
auto merge of #11879 : thestinger/rust/frame-pointer, r=alexcrichton

This is still used for Rust code (`Options.NoFramePointerElim = true`).