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.

targets/avr.json

@@ -13,6 +13,7 @@
 		"-Wl,--gc-sections"
 	],
 	"extra-files": [
+		"src/runtime/gc_avr.S",
 		"src/runtime/scheduler_avr.S"
 	]
 }

targets/cortex-m.json

@@ -25,6 +25,7 @@
 	],
 	"extra-files": [
 		"src/device/arm/cortexm.s",
+		"src/runtime/gc_arm.S",
 		"src/runtime/scheduler_cortexm.S"
 	],
 	"gdb": "gdb-multiarch"

targets/gameboy-advance.json

@@ -25,7 +25,7 @@
 	"linkerscript": "targets/gameboy-advance.ld",
 	"extra-files": [
 		"targets/gameboy-advance.s",
-		"src/runtime/scheduler_gba.S"
+		"src/runtime/gc_arm.S"
 	],
 	"gdb": "gdb-multiarch",
 	"emulator": ["mgba", "-3"]

targets/riscv.json

@@ -18,7 +18,7 @@
 	],
 	"extra-files": [
 		"src/device/riscv/start.S",
-		"src/runtime/scheduler_tinygoriscv.S",
+		"src/runtime/gc_riscv.S",
 		"src/device/riscv/handleinterrupt.S"
 	],
 	"gdb": "riscv64-unknown-elf-gdb"