Use correct names for __ibm128 if long double is IEEE 128-bit.

Use correct names for __ibm128 if long double is IEEE 128-bit.

If you are on a PowerPC system where the default long double is IEEE
128-bit, GCC will use the wrong names for some of the conversion functions
for the __ibm128 type.

What is happening is when the defult long double is IEEE 128-bit, the
various convert, truncation, and extend functions did not specify a
default name for the conversion.  The machine indepentent portions of the
compiler would construct a call with an 'if' name (IFmode being the mode
for IBM 128-bit floating point).  This patch specifies to use the
tradiational 'tf' name for these conversion functions.

2022-01-28  Michael Meissner  <meissner@the-meissners.org>

gcc/
	PR target/104253
	* config/rs6000/rs6000.cc (init_float128_ibm): Use the TF names
	for builtin conversions between __ibm128 and DImode when long
	double uses the IEEE 128-bit format.

gcc/testsuite/
	PR target/104253
	* gcc.target/powerpc/pr104253.c: New test.