runtime: copy stack scan assembly for GBA

The GC stack scanning code was implemented in the Cortex-M assembly, which meant that it was not available on the GBA which is pre-cortex.
This change adds a copy of the relevant code into a new asembly file which is used on the GBA.

src/runtime/scheduler_gba.S

@@ -0,0 +1,19 @@
+.section .text.tinygo_scanCurrentStack
+.global  tinygo_scanCurrentStack
+.type    tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Save callee-saved registers onto the stack.
+    mov r0, r8
+    mov r1, r9
+    mov r2, r10
+    mov r3, r11
+    push {r0-r3, lr}
+    push {r4-r7}
+
+    // Scan the stack.
+    mov r0, sp
+    bl tinygo_scanstack
+
+    // Restore stack state and return.
+    add sp, #32
+    pop {pc}