Avoid bashisms and switch to /bin/sh

Bash may not be installed on some systems by default, like FreeBSD.

Differential Revision: http://reviews.llvm.org/D5654


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@219231 91177308-0d34-0410-b5e6-96231b3b80d8