Don't use uname -o

Summary:
It's not required by the POSIX specification and OS X doesn't support
it; instead use uname -s.

Test Plan: V

Reviewers: O26 nofib, michalt, mpickering

Reviewed By: O26 nofib, michalt, mpickering

Subscribers: mpickering

GHC Trac Issues: #13711

Differential Revision: https://phabricator.haskell.org/D3594