[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>

board/gth2/lowlevel_init.S

@@ -413,7 +413,9 @@ noCacheJump:
 	j clearmem
 	nop
 
+#if 0
 	.globl	memtest
+#endif
 memtest:
 	/* Fill memory with address */
 	li	t0, 0x80000000
@@ -434,7 +436,9 @@ mt1:	lw	t2, 0(t0)
 	bne	t1, zero, mt1
 	nop
 	nop
+#if 0
 	.globl	clearmem
+#endif
 clearmem:
 		/* Clear memory */
 	li	t0, 0x80000000