Add support for cgo on openbsd/mips64.
Fixes#43005
Change-Id: I2386204f53fa984a01a9d89f0b6c96455768f326
Reviewed-on: https://go-review.googlesource.com/c/go/+/275896
Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>