[clang] Add powerpc64le-none-linux-gnu to gnu toolchain for PPC64

While trying to compile clang and openmp with a freshly built clang with the gcc/7.4.0
toolchain on the Summit supercomputer I face some error because of the triple under which
the GCC toolchain is installed was not present in for PPC64LE triples.
This patch add the powerpc64le-none-linux-gnu used on system like Summit and Ascent.

Reviewed By: jdenny

Differential Revision: https://reviews.llvm.org/D94261