compiler: add the //go:noinline pragma

This is directly useful to avoid some unsafety around runtime.alloc and
should be useful in general.

This pragma has the same form as in the main Go compiler:
https://github.com/golang/go/issues/12312

src/runtime/gc_conservative.go

@@ -203,6 +203,7 @@ func init() {
 
 // alloc tries to find some free space on the heap, possibly doing a garbage
 // collection cycle if needed. If no space is free, it panics.
+//go:noinline
 func alloc(size uintptr) unsafe.Pointer {
 	if size == 0 {
 		return unsafe.Pointer(&zeroSizedAlloc)