Make shell redirection construct portable

Summary:
NetBSD shell sh(1) does not support ">& /dev/null" construct.
This is bashism. The portable and POSIX solution is to use:
"> /dev/null 2>&1".

This change fixes 22 Unexpected Failures on NetBSD/amd64
for the "check-llvm" target.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, dim, rnk

Reviewed By: joerg, rnk

Subscribers: rnk, davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D35277

llvm-svn: 307789