mk: remove -fno-omit-frame-pointer

This is still used for Rust code (`Options.NoFramePointerElim = true`).
enable fp-elim when debug info is disabled

This can almost be fully disabled, as it no longer breaks retrieving a
backtrace on OS X as verified by @alexcrichton. However, it still
breaks retrieving the values of parameters. This should be fixable in
the future via a proper location list...

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

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

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Prefix _ to unused variables.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Append ; to #[allow(dead_code)].

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>