rs6000: Fix up __SIZEOF_{FLOAT,IBM}128__ defines [PR99708]

rs6000: Fix up __SIZEOF_{FLOAT,IBM}128__ defines [PR99708]

As mentioned in the PR, right now on powerpc* __SIZEOF_{FLOAT,IBM}128__
macros are predefined unconditionally, because {ieee,ibm}128_float_type_node
is always non-NULL, doesn't reflect whether __ieee128 or __ibm128 are
actually supported or not.

Based on patch review discussions, the following patch:
1) allows __ibm128 to be used in the sources even when !TARGET_FLOAT128_TYPE,
   as long as long double is double double
2) ensures ibm128_float_type_node is non-NULL only if __ibm128 is supported
3) ensures ieee128_float_type_node is non-NULL only if __ieee128 is supported
   (aka when TARGET_FLOAT128_TYPE)
4) predefines __SIZEOF_IBM128__ only when ibm128_float_type_node != NULL
5) newly predefines __SIZEOF_IEEE128__ if ieee128_float_type_node != NULL
6) predefines __SIZEOF_FLOAT128__ whenever ieee128_float_type_node != NULL
   and __float128 macro is predefined to __ieee128
7) removes ptr_*128_float_type_node which nothing uses
8) in order not to ICE during builtin initialization when
   ibm128_float_type_node == NULL, uses long_double_type_node as fallback
   for the __builtin_{,un}pack_ibm128 builtins
9) errors when those builtins are called used when
   ibm128_float_type_node == NULL (during their expansion)
10) moves the {,un}packif -> {,un}packtf remapping for these builtins in
    expansion earlier, so that we don't ICE on them if not -mabi=ieeelongdouble

2022-03-10  Jakub Jelinek  <jakub@redhat.com>

	PR target/99708
	* config/rs6000/rs6000.h (enum rs6000_builtin_type_index): Remove
	RS6000_BTI_ptr_ieee128_float and RS6000_BTI_ptr_ibm128_float.
	(ptr_ieee128_float_type_node, ptr_ibm128_float_type_node): Remove.
	* config/rs6000/rs6000-builtin.cc (rs6000_type_string): Return
	"**NULL**" if type_node is NULL first.  Handle
	ieee128_float_type_node.
	(rs6000_init_builtins): Don't initialize ptr_ieee128_float_type_node
	and ptr_ibm128_float_type_node.  Set ibm128_float_type_node and
	ieee128_float_type_node to NULL rather than long_double_type_node if
	they aren't supported.  Do support __ibm128 even if
	!TARGET_FLOAT128_TYPE when long double is double double.
	(rs6000_expand_builtin): Error if bif_is_ibm128 and
	!ibm128_float_type_node.  Remap RS6000_BIF_{,UN}PACK_IF to
	RS6000_BIF_{,UN}PACK_TF much earlier and only use bif_is_ibm128 check
	for it.
	* config/rs6000/rs6000-c.cc (rs6000_target_modify_macros): Define
	__SIZEOF_FLOAT128__ here and only iff __float128 macro is defined.
	(rs6000_cpu_cpp_builtins): Don't define __SIZEOF_FLOAT128__ here.
	Define __SIZEOF_IBM128__=16 if ieee128_float_type_node is non-NULL.
	Formatting fix.
	* config/rs6000/rs6000-gen-builtins.cc: Document ibm128 attribute.
	(struct attrinfo): Add isibm128 member.
	(TYPE_MAP_SIZE): Remove.
	(type_map): Use [] instead of [TYPE_MAP_SIZE].  For "if" use
	ibm128_float_type_node only if it is non-NULL, otherwise fall back
	to long_double_type_node.  Remove "pif" entry.
	(parse_bif_attrs): Handle ibm128 attribute and print it for debugging.
	(write_decls): Output bif_ibm128_bit and bif_is_ibm128.
	(write_type_node): Use sizeof type_map / sizeof type_map[0]
	instead of TYPE_MAP_SIZE.
	(write_bif_static_init): Handle isibm128.
	* config/rs6000/rs6000-builtins.def: Document ibm128 attribute.
	(__builtin_pack_ibm128, __builtin_unpack_ibm128): Add ibm128
	attribute.

	* gcc.dg/pr99708.c: New test.
	* gcc.target/powerpc/pr99708-2.c: New test.
	* gcc.target/powerpc/convert-fp-128.c (mode_kf): Define only if
	__FLOAT128_TYPE__ is defined.