[ARM] Drop "alloc" flag for the .stack segment.

Some linkers obey the linker script and make .stack unallocatable,
others obey the flags from the object files.  Dropping "a" should
make the end result deterministic in all cases.

arch/arm/boot/compressed/head.S

@@ -664,5 +664,5 @@ memdump:	mov	r12, r0
 reloc_end:
 
 		.align
-		.section ".stack", "aw"
+		.section ".stack", "w"
 user_stack:	.space	4096