cmd/asm: add support for LDREXB/STREXB
These are 8-bit ARM Load/Store atomics and are available starting from armv6k.
See https://developer.arm.com/documentation/dui0379/e/arm-and-thumb-instructions/strex
For #69735
Change-Id: I12623433c89070495c178208ee4758b3cdefd368
GitHub-Last-Rev:
d6a797836af1dccdcc6e6554725546b386d01615
GitHub-Pull-Request: golang/go#69959
Cq-Include-Trybots: luci.golang.try:gotip-linux-arm
Reviewed-on: https://go-review.googlesource.com/c/go/+/621395
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>