[MIPS] Introduce a consistent style for vmlinux.lds.

This style will be consitent with all other arch's - soon.

In addition:
- Moved a few labels inside brackets for the sections they specify
  to prevent that linker alignmnet made them point before the section start

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>