IBM zSystems: Save argument registers to the stack -mpreserve-args

IBM zSystems: Save argument registers to the stack -mpreserve-args

This adds support for preserving the content of parameter registers to
the stack and emit CFI for it. This useful for applications which want
to implement their own stack unwinding and need access to function
arguments.

With the -mpreserve-args option GPRs and FPRs are save to the stack
slots which are reserved for stdargs in the register save area.

gcc/ChangeLog:

	* config/s390/s390.cc (s390_restore_gpr_p): New function.
	(s390_preserve_gpr_arg_in_range_p): New function.
	(s390_preserve_gpr_arg_p): New function.
	(s390_preserve_fpr_arg_p): New function.
	(s390_register_info_stdarg_fpr): Rename to ...
	(s390_register_info_arg_fpr): ... this. Add -mpreserve-args handling.
	(s390_register_info_stdarg_gpr): Rename to ...
	(s390_register_info_arg_gpr): ... this. Add -mpreserve-args handling.
	(s390_register_info): Use the renamed functions above.
	(s390_optimize_register_info): Likewise.
	(save_fpr): Generate CFI for -mpreserve-args.
	(save_gprs): Generate CFI for -mpreserve-args. Drop return value.
	(s390_emit_prologue): Adjust to changed calling convention of save_gprs.
	(s390_optimize_prologue): Likewise.
	* config/s390/s390.opt: New option -mpreserve-args

gcc/testsuite/ChangeLog:

	* gcc.target/s390/preserve-args-1.c: New test.
	* gcc.target/s390/preserve-args-2.c: New test.