Map long double built-ins correctly with IEEE 128-bit long double.

Map long double built-ins correctly with IEEE 128-bit long double.

The PowerPC has two different 128-bit long double types, one that uses a pair
of doubles to get more mantissa range, and the other using the IEEE 128-bit
754R binary floating point format.  The pair of doubles has been used as the
traditional format, and we are in the process of moving to allow an
implementation to switch to using IEEE 128-bit floating point.  The GLIBC and
LIBSTDC++ libraries have been modified to have functions using the two
different formats in their libraries with different names.

This patch goes through all of the built-in functions that either take long
double arguments or return long double, and changes the name from the
traditional name to the IEEE 128-bit name.  The minimum GLIBC version to
support IEEE 128-bit floating point is 2.32.

The names changed are:

    *	<name>l is usually mapped to __<name>ieee128;
    *	<extra>printf is mapped to __<extra>printfieee128; (and)
    *	<extra>scanf is mapped to __isoc99_<extra>scanfieee128.

A few functions have different mappings:

    *	dreml		=> __remainderieee128;
    *	gammal		=> __lgammaieee128;
    *	gammal_r	=> __lgammaieee128_r;
    *	lgammal_r	=> __lgammaieee128_r;
    *	nexttoward	=> __nexttoward_to_ieee128;
    *	nexttowardf	=> __nexttowardf_to_ieee128;
    *	nexttowardl	=> __nexttowardl_to_ieee128;
    *	pow10l		=> __exp10ieee128;
    *	scalbl		=> __scalbieee128;
    *	significandl	=> __significandieee128; (and)
    *	sincosl		=> __sincosieee128.

gcc/
2021-01-28  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000.c (rs6000_mangle_decl_assembler_name): Add
	support for mapping built-in function names for long double
	built-in functions if long double is IEEE 128-bit.

gcc/testsuite/
2021-01-28  Michael Meissner  <meissner@linux.ibm.com>

	* gcc.target/powerpc/float128-longdouble-math.c: New test.
	* gcc.target/powerpc/float128-longdouble-stdio.c: New test.
	* gcc.target/powerpc/float128-math.c: Adjust test for new name
	being generated.  Add support for running test on power10.  Add
	support for running if long double defaults to 64-bits.