Add Python binary path to CMake arguments for the clang-ppc64le-linux builder

[bsaleil]

Summary

LLVM now requires Python 3.6 which is not available in the official Ubuntu 16.04 repositories. This patch adds the path to a manually built Python 3.6 to the CMake arguments of the clang-ppc64le-linux builder.

[gkistanova]

LGTM

[bsaleil]

I forgot to link the commit with this differential, but the patch has been committed: https://github.com/llvm/llvm-zorg/commit/45c4f238dc6fd9855b8578aa3ca3b8db336efb7e

buildbot/osuosl/master/config/builders.py

# Clang builders.
     'tags'  : ["clang", "ppc", "ppc64le"],
     'workernames' : ["ppc64le-clang-test"],
     'builddir': "clang-ppc64le",
     'factory' : ClangBuilder.getClangCMakeBuildFactory(
                     clean=False,
                     checkout_lld=False,
                     useTwoStage=False,
                     stage1_config='Release',
-                    extra_cmake_args=["-DLLVM_ENABLE_ASSERTIONS=ON"])},
+                    extra_cmake_args=[
+                        "-DLLVM_ENABLE_ASSERTIONS=ON",
+                        "-DPYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3"])},
 
     {'name' : "clang-ppc64le-rhel",
     'tags'  : ["clang", "ppc", "ppc64le"],
     'workernames' : ["ppc64le-clang-rhel-test"],
     'builddir': "clang-ppc64le-rhel",
     'factory' : ClangBuilder.getClangCMakeBuildFactory(clean=False,
                     checkout_clang_tools_extra=True,
                     checkout_compiler_rt=False,