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/arch_amd64.go

@@ -1,5 +1,7 @@
 package runtime
 
+import "device"
+
 const GOARCH = "amd64"
 
 // The bitness of the CPU (e.g. 8, 32, 64).
@@ -12,4 +14,6 @@ func align(ptr uintptr) uintptr {
 	return (ptr + 15) &^ 15
 }
 
-func getCurrentStackPointer() uintptr
+func getCurrentStackPointer() uintptr {
+	return device.AsmFull("movq %rsp, {}", nil)
+}