r18 (accessed as x18 and w18) is the platform register. The ABI testing framework cannot touch it, but we can statically check that our assembly leaves it alone. Also fix a comment which cited the wrong register. Change-Id: Iba2714eef5db19e2e93a6838d12a4e7c9011cc67 Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/35764 Reviewed-by: Adam Langley <agl@google.com>