Revert r204877 "Allow the user to override the compiler used for testing"

Unbreak the sanitizer-x86_64-linux bot.

Change-Id: Ie03f1b93fae45c54b142003731ff4f5d4f6b62f8

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204886 91177308-0d34-0410-b5e6-96231b3b80d8