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_amd64.S

@@ -0,0 +1,29 @@
+#ifdef __ELF__
+.section .text.tinygo_scanCurrentStack
+.global tinygo_scanCurrentStack
+tinygo_scanCurrentStack:
+#else // Darwin
+.global _tinygo_scanCurrentStack
+_tinygo_scanCurrentStack:
+#endif
+    // Save callee-saved registers.
+    pushq %rbx
+    pushq %rbp
+    pushq %r12
+    pushq %r13
+    pushq %r14
+    pushq %r15
+
+    // Scan the stack.
+    subq $8, %rsp // adjust the stack before the call to maintain 16-byte alignment
+    movq %rsp, %rdi
+#ifdef __ELF__
+    callq tinygo_scanstack
+#else
+    callq _tinygo_scanstack // Darwin
+#endif
+
+    // Restore the stack pointer. Registers do not need to be restored as they
+    // were only pushed to be discoverable by the GC.
+    addq $56, %rsp
+    retq