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_386.S

@@ -0,0 +1,22 @@
+.section .text.tinygo_scanCurrentStack
+.global tinygo_scanCurrentStack
+.type tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Sources:
+    //   * https://stackoverflow.com/questions/18024672/what-registers-are-preserved-through-a-linux-x86-64-function-call
+    //   * https://godbolt.org/z/q7e8dn
+
+    // Save callee-saved registers.
+    pushl %ebx
+    pushl %esi
+    pushl %edi
+    pushl %ebp
+
+    // Scan the stack.
+    pushl %esp
+    calll tinygo_scanstack
+
+    // Restore the stack pointer. Registers do not need to be restored as they
+    // were only pushed to be discoverable by the GC.
+    addl $20, %esp
+    retl