runtime: correct sigfwd on openbsd/mips64
runtime: correct sigfwd on openbsd/mips64

Position independent code expects that R25 (aka $t9) contains the address of the
called function. As such, use R25 when calling from sigfwd.

Change-Id: I66b2b9bfa1f1bb983c7385eb2eaa19d9cd87d9fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/275893
Trust: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,golang/go,golang/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,golang/go,cloudflare/go