runtime,syscall: convert syscall on openbsd/arm to libc

runtime,syscall: convert syscall on openbsd/arm to libc

Convert the syscall package on openbsd/arm to use libc rather than performing
direct system calls.

Updates #36435

Change-Id: Iff3a91c959292cbf4e0a09c7fd34efc8e88ff83f
Reviewed-on: https://go-review.googlesource.com/c/go/+/315793
Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>