arm: Add a W() macro for use in kernel assembler.

Some kernel assembly code uses a W macro to optionally add a .w to
instructions that need to be 32 bit thumb. The gnu assembler doesn't seem to
need the .w and won't assemble if it's provided.

Change-Id: I0a288177788b5c61810ee7bd3d2debea66835de2
Signed-off-by: Gabe Black <gabeblack@chromium.org>
Reviewed-on: http://review.coreboot.org/3759
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/armv7/include/assembler.h

@@ -58,3 +58,5 @@
  * Cache aligned
  */
 #define CALGN(code...) code
+
+#define W(instr)	instr