MIPS: Introduce --gc-sections for MIPS

All architectures but MIPS are using --gc-sections on final linking.
This patch introduces that feature for MIPS to reduce the memory and
flash footprint.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Thomas Lange <thomas@corelatus.se>
Cc: Vlad Lungu <vlad.lungu@windriver.com>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>