runtime,syscall: simplify openbsd related build tags

openbsd/mips64 is now the only openbsd port that uses non-libc syscall - revise
build tags to reflect this.

Update #36435

Change-Id: I357b2dd2926d058e25e618fcca42c388587598a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/317919


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