R600/SI: Remove another VALU pattern

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219988 91177308-0d34-0410-b5e6-96231b3b80d8
Add our own copy of the find_executable function to cope with installations
that do not have the distutils.spawn package. Should hopefully fix the
aarch64 buildbot.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219991 91177308-0d34-0410-b5e6-96231b3b80d8