runtime: convert openbsd/amd64 locking to libc

Switch openbsd/amd64 to locking via libc, rather than performing direct
system calls.

Update #36435

Change-Id: I5e92bd70ce557b78ff385577088a9775cc468ea9
Reviewed-on: https://go-review.googlesource.com/c/go/+/270378


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