Make -fzero-call-used-regs work on the SPARC

Make -fzero-call-used-regs work on the SPARC

This contains both a generic fixlet for targets implementing the leaf
register optimization (SPARC and Xtensa) and the implementation of the
target hook TARGET_ZERO_CALL_USED_REGS which is needed to make this work
on the SPARC.

gcc/ChangeLog:
	* function.c (gen_call_used_regs_seq): In a function subject to the
	leaf register optimization, skip registers that are not present.
	* config/sparc/sparc.c (TARGET_ZERO_CALL_USED_REGS): Define to...
	(sparc_zero_call_used_regs): ...this.  New function.