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