Makefile: Build uVisor for M3, even on M4s

Currently, uVisor builds with `-mcpu=cortex-m4`, even on Cortex M3 CPUs.
This can lead to using unavailable instructions. Build uVisor using the
least common denominator architecture. This avoids using unavailable
instructions when targeting Cortex M3 CPUs.

Author: Patater

Makefile

@@ -140,7 +140,7 @@ ifeq ("$(PROGRAM_VERSION)","")
 PROGRAM_VERSION:='unknown'
 endif
 
-FLAGS_CM4:=-mcpu=cortex-m4 -march=armv7e-m -mthumb
+FLAGS_CM4:=-mcpu=cortex-m3 -march=armv7-m -mthumb
 
 LDFLAGS:=\
         $(FLAGS_CM4) \