armv7: Use -march=armv7-a and thereby enable Thumb-2

Enable -march=armv7-a for armv7 platforms if the tool-chain
supports it. This in turn results in Thumb-2 code generated
for these platforms if CONFIG_SYS_THUMB_BUILD is enabled.

Signed-off-by: Aneesh V <aneesh@ti.com>

arch/arm/cpu/armv7/config.mk

@@ -22,8 +22,11 @@
 #
 PLATFORM_RELFLAGS += -fno-common -ffixed-r8 -msoft-float
 
-# Make ARMv5 to allow more compilers to work, even though its v7a.
-PLATFORM_CPPFLAGS += -march=armv5
+# If armv7-a is not supported by GCC fall-back to armv5, which is
+# supported by more tool-chains
+PF_CPPFLAGS_ARMV7 := $(call cc-option, -march=armv7-a, -march=armv5)
+PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_ARMV7)
+
 # =========================================================================
 #
 # Supply options according to compiler version