[PATCH,RS6000 1/5] Clean-up MASK_<xxxx> and RS6000_BTM_<xxxx> definitions.

[PATCH,RS6000 1/5] Clean-up MASK_<xxxx> and RS6000_BTM_<xxxx> definitions.

This patch removes the defines that are no longer used, and
updates the comment for the set of MASK_<xxxx> defines.

This patch removes the defines for
MASK_REGNAMES, MASK_PROTOTYPE, RS6000_BTM_ALWAYS, RS6000_BTM_COMMON.

gcc/
	* config/rs6000/rs6000.h (RS6000_BTM_COMMON, RS6000_BTM_ALWAYS,
	MASK_REGNAMES, OPTION_MASK_REGNAMES, MASK_PROTOTYPE,
	OPTION_MASK_PROTOTYPE, MASK_UPDATE, OPTION_MASK_UPDATE): Remove.