[build][toolchain] Fix error with ARCH_arm_TOOLCHAIN_PREFIX usage

If ARCH_arm_TOOLCHAIN_PREFIX was set then by default the makefile would
not set FOUNDTOOL and error out on build claiming no toolchain was found.
This change simply adds a check for gcc in the case that
ARCH_arm_TOOLCHAIN_PREFIX exists.

arch/arm/toolchain.mk

@@ -42,6 +42,8 @@ ifneq (,$(findstring arm-linux-gnueabi-,$(FOUNDTOOL)))
 endif
 endif # arm-linux-gnueabi-
 
+else
+FOUNDTOOL=$(shell which $(ARCH_arm_TOOLCHAIN_PREFIX)gcc)
 endif # ARCH_arm_TOOLCHAIN_PREFIX
 
 ifeq ($(FOUNDTOOL),)