runtime: convert openbsd/arm64 locking to libc

Switch openbsd/arm64 to locking via libc, rather than performing direct
system calls.

Update #36435

Change-Id: I2f30432c4bc232224cf87dca750665b8c40c7b72
Reviewed-on: https://go-review.googlesource.com/c/go/+/286813


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>