In support of the changes required for #8609, it was suggested that
syscall.getwd() be updated to work on Solaris first since the runtime
uses it and today it's unimplemented.

Fixes #12507

Change-Id: Ifb58ac9db8540936d5685c2c58bdc465dbc836cb
Reviewed-on: https://go-review.googlesource.com/14420Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>