runtime: use BX instead of R15 in race detector

If the race detector were runnable in dynamic linking mode,
then R15 would get clobbered. I don't think it is, so maybe
not a problem, but can't hurt to clean it up.

It also lets CL 283474 pass cleanly when checking the whole stdlib
(together with CL 288452).

Change-Id: I5a5021ecc7e7b8bed1cd3a7067c39b24c09e0783
Reviewed-on: https://go-review.googlesource.com/c/go/+/289270


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>