runtime: on arm32, detect whether we have sync instructions

runtime: on arm32, detect whether we have sync instructions

Make the choice of using these instructions dynamic (triggered by cpu
feature detection) rather than static (trigered by GOARM setting).

if GOARM>=7, we know we have them.
For GOARM=5/6, dynamically dispatch based on auxv information.

Update #17082
Update #61588

Change-Id: I8a50481d942f62cf36348998a99225d0d242f8af
Reviewed-on: https://go-review.googlesource.com/c/go/+/525637
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>