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.

targets/nintendoswitch.json

@@ -27,6 +27,7 @@
   "linkerscript": "targets/nintendoswitch.ld",
   "extra-files": [
     "targets/nintendoswitch.s",
+    "src/runtime/gc_arm64.S",
     "src/runtime/runtime_nintendoswitch.s"
   ]
 }