R18 seems reserved on Apple platforms.

May fix darwin/arm64 build.

Change-Id: Ia2c1de550a64827c85a64affa53b94c62aacce8e
Reviewed-on: https://go-review.googlesource.com/98896
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>