qemu-arm: Use gchelper code to get registers for GC scanning.

ports/qemu-arm/Makefile

@@ -13,6 +13,7 @@ ifeq ($(BOARD),netduino2)
 CFLAGS += -mthumb -mcpu=cortex-m3 -mfloat-abi=soft
 CFLAGS += -DQEMU_SOC_STM32
 LDSCRIPT = stm32.ld
+SRC_BOARD_O = lib/utils/gchelper_m3.o
 endif
 
 ifeq ($(BOARD),microbit)
@@ -20,12 +21,14 @@ CFLAGS += -mthumb -mcpu=cortex-m0 -mfloat-abi=soft
 CFLAGS += -DQEMU_SOC_NRF51
 LDSCRIPT = nrf51.ld
 QEMU_EXTRA = -global nrf51-soc.flash-size=1048576 -global nrf51-soc.sram-size=262144
+SRC_BOARD_O = lib/utils/gchelper_m0.o
 endif
 
 ifeq ($(BOARD),mps2-an385)
 CFLAGS += -mthumb -mcpu=cortex-m3 -mfloat-abi=soft
 CFLAGS += -DQEMU_SOC_MPS2
 LDSCRIPT = mps2.ld
+SRC_BOARD_O = lib/utils/gchelper_m3.o
 endif
 
 CROSS_COMPILE = arm-none-eabi-
@@ -95,6 +98,7 @@ LIB_SRC_C += $(addprefix lib/,\
 OBJ_COMMON =
 OBJ_COMMON += $(PY_O)
 OBJ_COMMON += $(addprefix $(BUILD)/, $(SRC_COMMON_C:.c=.o))
+OBJ_COMMON += $(addprefix $(BUILD)/, $(SRC_BOARD_O))
 OBJ_COMMON += $(addprefix $(BUILD)/, $(LIB_SRC_C:.c=.o))
 
 OBJ_RUN =

ports/qemu-arm/test_main.c

@@ -11,6 +11,7 @@
 #include "py/stackctrl.h"
 #include "py/gc.h"
 #include "py/mperrno.h"
+#include "lib/utils/gchelper.h"
 
 #include "tinytest.h"
 #include "tinytest_macros.h"
@@ -33,9 +34,8 @@ void gc_collect(void) {
     gc_collect_start();
 
     // get the registers and the sp
-    // TODO get registers
-    volatile mp_uint_t dummy;
-    void *sp = (void*)&dummy;
+    uintptr_t regs[10];
+    uintptr_t sp = gc_helper_get_regs_and_sp(regs);
 
     // trace the stack, including the registers (since they live on the stack in this function)
     gc_collect_root((void**)sp, ((uint32_t)MP_STATE_THREAD(stack_top) - (uint32_t)sp) / sizeof(uint32_t));