* calls.c (store_one_arg): Set default alignment for BLKmode arguments
	to BITS_PER_UNIT when ARGS_GROW_DOWNWARD and the padding direction is
	downward.
	* function.c (pad_below):  Always compile.
	(locate_and_pad_parm): If defined ARGS_GROW_DOWNWARD, pad argument to
	alignment when it is not in a register or REG_PARM_STACK_SPACE is true.
	Pad below when the argument is not in a register and the padding
	direction is downward.
	* pa-64.h (MUST_PASS_IN_STACK): Move define to pa.h.
	(PAD_VARARGS_DOWN): Define.
	* pa.c (function_arg_padding): Revise padding directions to make them
	compatible with the 32 and 64-bit runtime architecture documentation.
	(hppa_va_arg):  Add code to handle variable and size zero arguments
	passed by reference on TARGET_64BIT.  Reformat.
	(function_arg): Use a PARALLEL for BLKmode and aggregates args on
	TARGET_64BIT.  Use a DImode PARALLEL for BLKmode args 5 to 8 bytes
	wide when !TARGET_64BIT.  Move forward check for mode==VOIDmode.
	Add comments.
	* pa.h (MAX_PARM_BOUNDARY): Correct define for TARGET_64BIT.
	(RETURN_IN_MEMORY): Return size zero types in memory.
	(FUNCTION_VALUE): Return TFmode in general registers.
	(MUST_PASS_IN_STACK): Define.
	(FUNCTION_ARG_BOUNDARY): Simplify.
	(FUNCTION_ARG_PASS_BY_REFERENCE): Pass variable and zero sized types
	by reference.
	(FUNCTION_ARG_CALLEE_COPIES): Define to FUNCTION_ARG_PASS_BY_REFERENCE.

From-SVN: r57226