cmd/gc: fix racewalk after writebarrierfat was renamed to typedmemmove

Renaming the function broke the race detector since it looked for the
name, didn't find it anymore and didn't insert the necessary
instrumentation.

Change-Id: I11fed6e807cc35be5724d26af12ceff33ebf4f7b
Reviewed-on: https://go-review.googlesource.com/2661


Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>