runtime: change tfork behaviour to unbreak openbsd/mips64
Currently, tfork on openbsd/mips64 returns the thread ID on success and
a negative error number on error. In CL#447175, newosproc was changed
to assume that a non-zero value is an error - return zero on success to
match this expectation.
Change-Id: I955efad49b149146165eba3d05fe40ba75caa098
Reviewed-on: https://go-review.googlesource.com/c/go/+/451257 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Joedian Reid <joedian@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>