Update CFLAGS for clang compatibility

Update CFLAGS for clang compatibility

* Clang/llvm does not (yet) support -m(no-)spe, so make it gcc-only
* Clang now supports -msoft-float, and does not appear to recognize
  "-disable-ppc-float-in-variadic", which appears to have been a crutch until
  soft-float was implemented.  It's now implemented for both 32- and 64-bit.
* Clang/llvm use a 'medium' code model by default for powerpc64, supporting up
  to 4GB TOC, and does not support the '-mminimal-toc' option.  Given both of
  these, make -mminimal-toc gcc-only.

MFC after:	2 weeks