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

[clementval]

Summary

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.

[jdenny]

LGTM

clang/lib/Driver/ToolChains/Gnu.cpp

/*static*/ void Generic_GCC::GCCInstallationDetector::CollectLibDirsAndTriples(
 
   static const char *const PPC64LibDirs[] = {"/lib64", "/lib"};
   static const char *const PPC64Triples[] = {
       "powerpc64-linux-gnu", "powerpc64-unknown-linux-gnu",
       "powerpc64-suse-linux", "ppc64-redhat-linux"};
   static const char *const PPC64LELibDirs[] = {"/lib64", "/lib"};
   static const char *const PPC64LETriples[] = {
       "powerpc64le-linux-gnu", "powerpc64le-unknown-linux-gnu",
-      "powerpc64le-suse-linux", "ppc64le-redhat-linux"};
+      "powerpc64le-none-linux-gnu", "powerpc64le-suse-linux",
+      "ppc64le-redhat-linux"};
 
   static const char *const RISCV32LibDirs[] = {"/lib32", "/lib"};
   static const char *const RISCV32Triples[] = {"riscv32-unknown-linux-gnu",
                                                "riscv32-linux-gnu",
                                                "riscv32-unknown-elf"};
   static const char *const RISCV64LibDirs[] = {"/lib64", "/lib"};
   static const char *const RISCV64Triples[] = {"riscv64-unknown-linux-gnu",
                                                "riscv64-linux-gnu",