msp430: configure according to $(CC)

The checks for how to compile should
be with respect to the compiler used
for the build.

arch/cpu/msp430/Makefile.msp430

@@ -146,7 +146,7 @@ endif
 
 ### Checks for compiler version to enable 20-bit support
 ifndef IAR
-ifneq (,$(findstring 4.7.,$(shell msp430-gcc -dumpversion)))
+ifneq (,$(findstring 4.7.,$(shell $(CC) -dumpversion)))
 ifdef CPU_HAS_MSP430X
  ifeq ($(TARGET_MEMORY_MODEL),large)
   CFLAGS += -mmemory-model=$(TARGET_MEMORY_MODEL)