rs6000: Add "enabled" attribute

rs6000: Add "enabled" attribute

This adds the "enabled" attribute to the rs6000 backend.  It uses the
(new) "isa" attribute to automatically select which instruction
alternatives should be enabled.

For now it allows isa strings of "p5", "p6", "p7", meaning the
instructions introduced on that CPU, not requiring vectors; and "p7v",
"p8v", "p9v" for the same, but with vectors.

These are currently mapped to TARGET_POPCNTB, TARGET_CMPB,
TARGET_POPCNTD, TARGET_VSX, TARGET_P8_VECTOR, and TARGET_P9_VECTOR;
that will change to something a bit saner later.

	* config/rs6000/rs6000.md (isa): New attribute.
	(enabled): New attribute.

From-SVN: r271360