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 set of patches fixes places in the compiler where it was assumed to use
__ibm128 we needed to have IEEE 128-bit support enabled.  A future patch will
implement the change to create the __ibm128 type if either we have IEEE 128-bit
floating point enabled, or the long double type is IBM 128-bit floating point.

2022-07-28   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs600.cc (init_float128_ibm): Remove checks for hardware
	floating point for the IBM 128-bit comparisons.
	(rs6000_init_libfuncs): Create IBM 128-bit floating point support even
	if we don't support IEEE 128-bit floating point.
	(rs6000_scalar_mode_supported_p): Allow __ibm128 even if we don't
	support _Float128.
	* config/rs6000/rs6000.h (FLOAT128_IBM_P): Remove checks for
	-mhard-float.
	(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.
	(extendtfif2): Allow conversion if we have __ibm128 but not IEEE 128-bit
	floating point.
	(trunckftf2): Likewise.