To support "SVC $0x80", which is needed for darwin/arm64.

Change-Id: I3b3f80791a1db4c2b7318f81a115972cd2237f00
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8769Reviewed-by: David Crawshaw <crawshaw@golang.org>