R15 is the real register. PC is a pseudo-register that we are making
illegal in this context as part of the grand assembly unification.

Change-Id: Ie0ea38ce7ef4d2cf4fcbe23b851a570fd312ce8d
Reviewed-on: https://go-review.googlesource.com/4966Reviewed-by: Minux Ma <minux@golang.org>