runtime: switch runtime to libc for openbsd/386

Use libc rather than performing direct system calls for the runtime on
openbsd/386.

Updates #36435

Change-Id: I0cd65368bc824c81f5f98ea24e4f82db5468b170
Reviewed-on: https://go-review.googlesource.com/c/go/+/287653


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>