reginfo: Add a global_reg_set

reginfo: Add a global_reg_set

A later patch wants to use the set of global registers as a HARD_REG_SET
rather than a bool/char array.  Most other arrays already have a
HARD_REG_SET counterpart, but this one didn't.

gcc/
	* hard-reg-set.h (global_reg_set): Declare.
	* reginfo.c (global_reg_set): New variable.
	(init_reg_sets_1, globalize_reg): Update it when globalizing
	registers.