Allow the user to override the compiler used for testing

Change-Id: Ida62c2d85f541260f505c3dbd5d71a4b2a6004f4

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204877 91177308-0d34-0410-b5e6-96231b3b80d8