runtime/cgo: add support for cgo on openbsd/mips64

[4a6f656c]

The openbsd/mips64 port does not currently support cgo.

This is a bug to track work to enable cgo on this port.

[cagedmantis]

/cc @ianlancetaylor

[gopherbot]

Change https://golang.org/cl/275894 mentions this issue: cmd/link/internal/loadelf: support additional ELF relocations on mips64

[gopherbot]

Change https://golang.org/cl/275896 mentions this issue: cmd/dist,runtime: support cgo on openbsd/mips64