runtime.memclr* functions have signatures

func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)
func memclrHasPointers(ptr unsafe.Pointer, n uintptr)

Update compiler's copy. Also teach gc/mkbuiltin.go to handle
unsafe.Pointer. The import statement and its support is not
really necessary, but just to make it look like real Go code.

Fixes #19185.

Change-Id: I251d02571fde2716d4727e31e04d56ec04b6f22a
Reviewed-on: https://go-review.googlesource.com/37257
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>