rs6000: Rename rs6000-builtin-new.def to rs6000-builtins.def

2021-12-02  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-builtin-new.def: Rename to...
	* config/rs6000/rs6000-builtins.def: ...this.
	* config/rs6000/rs6000-gen-builtins.c: Adjust header commentary.
	* config/rs6000/t-rs6000 (EXTRA_GTYPE_DEPS): Rename
	rs6000-builtin-new.def to rs6000-builtins.def.
	(rs6000-builtins.c): Likewise.