MIPS: Coding style cleanups on common assembly files

Fix style issues and alignments globally.  No logical changes.
- Replace C comments with AS line comments where possible
- Use ifndef where possible, rather than if !defined for simplicity
- An instruction executed in a delay slot is now indicated by a leading
  space, not by C comment

Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>