armv7: inform the assembler about thumb

In some toolchains, the assembler defaults to arm and gcc does not
pass automatically the option for switching to the thumb mode, so
we have to do it by hand.

Makefile.defs

@@ -1287,8 +1287,11 @@ ifeq	($(ARCH), arm7)
 		# if gcc 
 ifeq		($(CC_NAME), gcc)
 				C_DEFS+=-DCC_GCC_LIKE_ASM
-				#common stuff
+				#common stuff + inform the assembler about THUMB
+				# (some assemblers default to ARM, see
+				# https://wiki.ubuntu.com/ARM/Thumb2)
 				CFLAGS=-march=armv7-a -O9 -funroll-loops -fsigned-char \
+						-Wa,-mimplicit-it=thumb \
 						$(PROFILE)
 			#if gcc 4.5 or 4.2+
 ifeq (,$(strip $(filter-out 4.2+ 4.5,$(CC_SHORTVER))))