arm: use canonical sub mnemonic

Building some arm boards with older binutils may produce errors like this:

---8<---
crt0.S: Assembler messages:
crt0.S:70: Error: register expected, not '#(184)' -- `sub sp,#(184)'
--->8---

Use canonical version of the subtract mnemonic to avoid those issues.

Reported-by: Alexey Smishlayev <alexey@xtech2.lv>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/arch/arm/lib/crt0.S b/arch/arm/lib/crt0.S
index ac54b93..dfc2de9 100644
--- a/arch/arm/lib/crt0.S
+++ b/arch/arm/lib/crt0.S
@@ -67,7 +67,7 @@
 	ldr	sp, =(CONFIG_SYS_INIT_SP_ADDR)
 #endif
 	bic	sp, sp, #7	/* 8-byte alignment for ABI compliance */
-	sub	sp, #GD_SIZE	/* allocate one GD above SP */
+	sub	sp, sp, #GD_SIZE	/* allocate one GD above SP */
 	bic	sp, sp, #7	/* 8-byte alignment for ABI compliance */
 	mov	r9, sp		/* GD is above SP */
 	mov	r0, #0