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_stack_raw.go

@@ -1,5 +1,5 @@
 // +build gc.conservative gc.extalloc
-// +build baremetal
+// +build !wasm
 
 package runtime