[MIPS] board/gth2/lowlevel_init.S: Fix a build warning

lowlevel_init.S: Assembler messages:
lowlevel_init.S:413: Warning: Pretending global symbol used as branch target is local.

Looking at codes, the `memtest' and `clearmem' are intentional mixed
use of `global symbols' and `label' for debugging purpose. To make it
build, just disable global-symbols-use for now. As a result `memtest'
still remains as unused, but leave it be...

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>