Allow __ibm128 on older PowerPC systems.

Allow __ibm128 on older PowerPC systems.

In a previous patch, I added code to ibm-ldouble.c to use __builtin_pack_ibm128
if long double is IEEE 128-bit and continue to use __builtin_pack_longdouble
if long double is IBM extended double.  This code was needed because
__builtin_pack_ibm128 is not available unless the __ibm128 keyword is
availabe.  In the current code, __ibm128 is only enabled if we have support for
both IBM and IEEE 128-bit long double.

Segher suggested that instead I should make __ibm128, __builtin_pack_ibm128,
and __builtin_unpack_ibm128 available on older systems that don't support IEEE
128-bit floating point but does support the IBM extended double floating point.

This patch changes the code so that __ibm128 is now exported if either
long double uses the IBM extended double format, or IEEE 128-bit floating
point is available.

I changed the internal built-in types from float128 to ibm128, since the
only built-in functions that use this are __builtin_pack_ibm128 and
__builtin_unpack_ibm128, and the new name matches the function.

gcc/
2021-04-28  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000-builtin.def (BU_IBM128_2): Rename
	RS6000_BTM_IBM128 from RS6000_BTM_FLOAT128.
	* config/rs6000/rs6000-call.c (rs6000_invalid_builtin): Update
	error message for __ibm128 built-in functions.
	(rs6000_init_builtins): Create the __ibm128 keyword on older
	systems where long double uses the IBM extended double format,
	even if they don't support IEEE 128-bit floating point.
	* config/rs6000/rs6000.c (rs6000_builtin_mask_calculate): Rename
	RS6000_BTM_IBM128 from RS6000_BTM_FLOAT128.
	(rs6000_builtin_mask_names): Rename RS6000_BTM_IBM128 from
	RS6000_BTM_FLOAT128.
	* config/rs6000/rs6000.h (TARGET_IBM128): New macro.
	(RS6000_BTM_IBM128): Rename from RS6000_BTM_FLOAT128.
	(RS6000_BTM_COMMON): Rename RS6000_BTM_IBM128 from
	RS6000_BTM_FLOAT128.