Fix native powerpc64 build of lib32 with in-tree GCC.

- This was broken by r300350 and r300885.
- Add some comments around the external GCC logic since it is spread out
  and in need of some cleanup.
- The problem was that X_COMPILER_TYPE is always defined from CC->XCC's
  default, so if /usr/bin/cc is GCC (as it is on native powerpc64) then
  X_COMPILER_TYPE was getting GCC and triggering the external logic in
  Makefile.libcompat.  It was intended to always provide -isystem with
  GCC since --sysroot is used into the lib32 sysroot which won't modify
  the header path without the -isystem.  The use of the libc++/std=c++11
  override was only intended to be used for external compilers though
  (more accurately GCC 4.8+ but that's a separate assumption to
  cleanup).  Apply the same logic from Makefile.inc1 to Makefile.libcompat
  to only add the libc++ override when needed for external compilers.

Pointyhat to:	bdrewery
Tested with:	native ppc64 (swills), universe, ppc64 xtoolchain,
		amd64 xtoolchain, sparc64 cross-build of ppc64 (host GCC 4.2)
Reported by:	andreast, swills
Approved by:	re (gjb)
Sponsored by:	EMC / Isilon Storage Division

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