Allow __ibm128 even if IEEE 128-bit floating point is not supported.

Allow __ibm128 even if IEEE 128-bit floating point is not supported.

This patch allows the use of the __ibm128 keyword on non-VSX systems.
Originally, the __ibm128 keyword was only enabled when the IEEE 128-bit
floating point is enabled.  Sometime back in the GCC 12 development period,
Segher asked that the __ibm128 keyword be allowed in older systems that don't
support IEEE 128-bit.  But at the time, stage 1 had closed for GCC 12, so I
deferred doing this change until GCC 13.  This patch allows __ibm128 to be used
if either IEEE 128-bit is enabled or long double used the IBM 128-bit format.

2022-09-09   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000-builtins.cc (rs6000_init_builtins): Enable using
	the__ibm128 keyword on systems that either use the 128-bit IBM long
	double format for long double or support IEEE 128-bit.
	* config/rs6000/rs6000.cc (rs6000_init_libfuncs): Create IBM 128-bit
	floating point support functions on systems that support the __ibm128
	keyword.
	(rs6000_scalar_mode_supported_p): Likewise.
	* config/rs6000/rs6000.h (TARGET_IBM128): New macro.
	* config/rs6000/rs6000.md (@extenddf<mode>2_fprs): Allow IFmode to be
	converted even if long double is not 128-bits.
	(extenddf<mode>2_vsx): Likewise.
	(extendiftf2):Allow conversion on systems that support the __ibm128
	keyword.
	(extendtfif2): Likewise.
	(trunciftf2): Likewise.
	(trunctfif2): Likewise.