runtime: syscall_forkx on Solaris can return error on success

The syscall_forkx function returns the value of errno even on success.  This can be a problem when using cgo where an atfork handler might be registered; if the atfork handler does something which causes errno to be set the caller of syscall_forkx can be misled into thinking the fork has failed.  This causes the various exec functions in the runtime package to hang.

Change-Id: Ia1842179226078a0cbbea33d541aa1187dc47f68
GitHub-Last-Rev: 4dc4db75c82a826da9a50c323b7e3ddfe46ed6c0
GitHub-Pull-Request: golang/go#36076
Reviewed-on: https://go-review.googlesource.com/c/go/+/210742


Reviewed-by: Ian Lance Taylor <iant@golang.org>