[MIPS] Do not discard extra debugging sections.

Leaving these sections is useful to some tools that look at the image, and
none of them are loaded into memory.  The .mdebug.abi64 section, in
particular, lets GDB recognize vmlinux.32 as an N64 program instead of
guessing that it is O32.

Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>