boot/shim: force arm mode instead of Thumb mode

Use OPTIMIZATIONS parameter to pass -marm to fix the following build
failure in Thumb mode:

/tmp/ccEjPM4h.s:941: Error: selected processor does not support `mrc p15,0,r2,c9,c13,0' in Thumb mode

CFLAGS can't be overridden as it is defined as:
CFLAGS		= $(FEATUREFLAGS) \
		  $(OPTIMIZATIONS) \
		  $(WARNFLAGS) \
		  $(if $(findstring clang,$(CC)),$(CLANG_WARNINGS)) \
		  $(ARCH_CFLAGS) \
		  $(WERRFLAGS) \
		  $(INCLUDES) \
		  $(DEFINES)

Fixes:
 - http://autobuild.buildroot.org/results/580156f89cfd72122fef07aa2fe37b4fdd4c316b
 - http://autobuild.buildroot.org/results/8980c0f422516c7263d8d0c9cc6123f30c4b7ee4

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 66a202325a98a48013bad70de29d88f733ce5903)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/shim/shim.mk

@@ -13,10 +13,19 @@ SHIM_CPE_ID_VENDOR = redhat
 SHIM_INSTALL_TARGET = NO
 SHIM_INSTALL_IMAGES = YES
 
+SHIM_CFLAGS = $(TARGET_CFLAGS)
 SHIM_MAKE_OPTS = \
 	ARCH="$(GNU_EFI_PLATFORM)" \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
-	DASHJ="-j$(PARALLEL_JOBS)"
+	DASHJ="-j$(PARALLEL_JOBS)" \
+	OPTIMIZATIONS="$(SHIM_CFLAGS)"
+
+# shim has some assembly function that is not present in Thumb mode:
+# Error: selected processor does not support `mrc p15,0,r2,c9,c13,0' in Thumb mode
+# so, we desactivate Thumb mode
+ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB),y)
+SHIM_CFLAGS += -marm
+endif
 
 define SHIM_BUILD_CMDS
 	$(TARGET_CONFIGURE_OPTS) $(MAKE) -C $(@D) $(SHIM_MAKE_OPTS)