{RFA] PowerPC e500mc64 core support

[David Edelsohn]

Edmar,

Creating TARGET_E500MC64 is incorrect, the macro should represent a
feature, not a processor.  The rs6000.c tests should use
PROCESSOR_E500MC64.

The change to floatdidf2 pattern does not make sense because it
already tests TARGET_POWERPC64 and e500mc64 sets MASK_POWERPC64,
similarly for the other di/df patterns.

I do not see any useful purpose for TARGET_E500MC64.

Should rs6000_emit_sCOND() call rs6000_emit_sISEL() instead of
modifying cstore<mode>4 ?

Thanks, David