[PowerPC] Support powerpc64le as a syntax-checking target.

This patch provides basic support for powerpc64le as an LLVM target.
However, use of this target will not actually generate little-endian
code.  Instead, use of the target will cause the correct little-endian
built-in defines to be generated, so that code that tests for
__LITTLE_ENDIAN__, for example, will be correctly parsed for
syntax-only testing.  Code generation will otherwise be the same as
powerpc64 (big-endian), for now.

The patch leaves open the possibility of creating a little-endian
PowerPC64 back end, but there is no immediate intent to create such a
thing.

The LLVM portions of this patch simply add ppc64le coverage everywhere
that ppc64 coverage currently exists.  There is nothing of any import
worth testing until such time as little-endian code generation is
implemented.  In the corresponding Clang patch, there is a new test
case variant to ensure that correct built-in defines for little-endian
code are generated.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187179 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Object/ELF.h

@@ -2796,7 +2796,8 @@ unsigned ELFObjectFile<ELFT>::getArch() const {
     return (ELFT::TargetEndianness == support::little) ?
            Triple::mipsel : Triple::mips;
   case ELF::EM_PPC64:
-    return Triple::ppc64;
+    return (ELFT::TargetEndianness == support::little) ?
+           Triple::ppc64le : Triple::ppc64;
   case ELF::EM_S390:
     return Triple::systemz;
   default: