runtime: move/refactor some GC-related code

Instead of putting tinygo_scanCurrentStack in scheduler_*.S files, put
them in dedicated files. The function tinygo_scanCurrentStack has
nothing to do with scheduling and so doesn't belong there. Additionally,
while scheduling code is made specific for the Cortex-M, the
tinygo_scanCurrentStack is generic to all ARM targets so this move
removes some duplication there.

Specifically:

  * tinygo_scanCurrentStack is moved out of scheduler_cortexm.S as it
    isn't really part of the scheduler. It is now gc_arm.S.
  * Same for the AVR target.
  * Same for the RISCV target.
  * scheduler_gba.S is removed, using gc_arm.S instead as it only
    contains tinygo_scanCurrentStack.

src/runtime/gc_avr.S

@@ -0,0 +1,52 @@
+.section .text.tinygo_scanCurrentStack
+.global tinygo_scanCurrentStack
+.type tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Save callee-saved registers.
+    push r29 // Y
+    push r28 // Y
+    push r17
+    push r16
+    push r15
+    push r14
+    push r13
+    push r12
+    push r11
+    push r10
+    push r9
+    push r8
+    push r7
+    push r6
+    push r5
+    push r4
+    push r3
+    push r2
+
+    // Scan the stack.
+    in  r24, 0x3d; SPL
+    in  r25, 0x3e; SPH
+#if __AVR_ARCH__ == 2 || __AVR_ARCH__ == 25
+    rcall tinygo_scanstack
+#else
+    call tinygo_scanstack
+#endif
+
+    // Restore callee-saved registers.
+    pop r2
+    pop r3
+    pop r4
+    pop r5
+    pop r6
+    pop r7
+    pop r8
+    pop r9
+    pop r10
+    pop r11
+    pop r12
+    pop r13
+    pop r14
+    pop r15
+    pop r16
+    pop r17
+    pop r28 // Y
+    pop r29 // Y