Add gchelper.s, to load registers and clean stack for GC.

stm/gchelper.s

+    .syntax unified
+    .cpu cortex-m4
+    .thumb
+    .text
+    .align  2
+
+@ void gc_helper_get_regs_and_clean_stack(r0=uint regs[10], r1=heap_end)
+    .global gc_helper_get_regs_and_clean_stack
+    .thumb
+    .thumb_func
+    .type   gc_helper_get_regs_and_clean_stack, %function
+gc_helper_get_regs_and_clean_stack:
+    @ store registers into given array
+    str     r4, [r0], #4
+    str     r5, [r0], #4
+    str     r6, [r0], #4
+    str     r7, [r0], #4
+    str     r8, [r0], #4
+    str     r9, [r0], #4
+    str     r10, [r0], #4
+    str     r11, [r0], #4
+    str     r12, [r0], #4
+    str     r13, [r0], #4
+
+    @ clean the stack from given pointer up to current sp
+    movs    r0, #0
+    mov     r2, sp
+    b       .entry
+.loop:
+    str     r0, [r1], #4
+.entry:
+    cmp     r1, r2
+    bcc     .loop
+    bx      lr
+
+    .size   gc_helper_get_regs_and_clean_stack, .-gc_helper_get_regs_and_clean_stack