gc: use raw stack access whenever possible

The only architecture that actually needs special support for scanning
the stack is WebAssembly. All others allow raw access to the stack with
a small bit of assembly. Therefore, don't manually keep track of all
these objects on the stack manually and instead just use conservative
stack scanning.

This results in a massive code size decrease in the affected targets
(only tested linux/amd64 for code size) - sometimes around 33%. It also
allows for future improvements such as using proper stackful goroutines.

src/runtime/gc_arm64.S

@@ -0,0 +1,23 @@
+.section .text.tinygo_scanCurrentStack
+.global tinygo_scanCurrentStack
+.type tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Sources:
+    //   * https://developer.arm.com/architectures/learn-the-architecture/armv8-a-instruction-set-architecture/procedure-call-standard
+    //   * https://godbolt.org/z/qrvrEh
+
+    // Save callee-saved registers.
+    stp     x29, x30, [sp, #-96]!
+    stp     x28, x27, [sp, #16]
+    stp     x26, x25, [sp, #32]
+    stp     x24, x23, [sp, #48]
+    stp     x22, x21, [sp, #64]
+    stp     x20, x19, [sp, #80]
+
+    // Scan the stack.
+    mov     x0, sp
+    bl      tinygo_scanstack
+
+    // Restore stack state and return.
+    ldp     x29, x30, [sp], #96
+    ret