samd/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/samd/Makefile

@@ -79,6 +79,8 @@ LIBSTDCPP_FILE_NAME = "$(shell $(CXX) $(CXXFLAGS) -print-file-name=libstdc++.a)"
 LDFLAGS += -L"$(shell dirname $(LIBSTDCPP_FILE_NAME))"
 endif
 
+MPY_CROSS_FLAGS += -march=$(MPY_CROSS_MCU_ARCH)
+
 SRC_C += \
 	mcu/$(MCU_SERIES_LOWER)/clock_config.c \
 	help.c \

ports/samd/mcu/samd21/mpconfigmcu.mk

@@ -1,5 +1,7 @@
 CFLAGS_MCU += -mtune=cortex-m0plus -mcpu=cortex-m0plus -msoft-float
 
+MPY_CROSS_MCU_ARCH = armv6m
+
 SRC_S += shared/runtime/gchelper_m0.s
 
 LIBM_SRC_C +=  $(addprefix lib/libm/,\

ports/samd/mcu/samd51/mpconfigmcu.mk

@@ -1,5 +1,7 @@
 CFLAGS_MCU += -mtune=cortex-m4 -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard
 
+MPY_CROSS_MCU_ARCH = armv7m
+
 MICROPY_VFS_LFS2 ?= 1
 MICROPY_VFS_FAT ?= 1
 FROZEN_MANIFEST ?= mcu/$(MCU_SERIES_LOWER)/manifest.py