Remove bashisms and switch to /bin/sh

On a default FreeBSD install Bash is not installed and fdescfs is not
mounted.  Use plain sh functionality instead.

Differential Revision: http://reviews.llvm.org/D5631


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@219333 91177308-0d34-0410-b5e6-96231b3b80d8