mimxrt/Makefile: Set MPY_CROSS_FLAGS.

Otherwise this port will be unable to freeze `@native`/`@viper` code.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/mimxrt/Makefile

@@ -366,6 +366,8 @@ endif
 
 CFLAGS += $(CFLAGS_EXTRA)
 
+MPY_CROSS_FLAGS += -march=armv7m
+
 # =============================================================================
 # Linker Flags
 # =============================================================================