Add saturating subtract built-ins.

Add saturating subtract built-ins.

This patch adds support for a saturating subtract built-in function that may be
added to a future PowerPC processor.  Note, if it is added, the name of the
built-in function may change before GCC 13 is released.  If the name changes,
we will submit a patch changing the name.

I also added support for providing dense math built-in functions, even though
at present, we have not added any new built-in functions for dense math.  It is
likely we will want to add new dense math built-in functions as the dense math
support is fleshed out.

I tested this patch on a little endian power10 system with long double using
the tradiational IBM double double format.  Assuming the other 6 patches for
-mcpu=future are checked in (or at least the first patch), can I check this
patch into the master branch for GCC 13.

2023-01-27   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000-builtin.cc (rs6000_invalid_builtin): Add support
	for flagging invalid use of future built-in functions.
	(rs6000_builtin_is_supported): Add support for future built-in
	functions.
	* config/rs6000/rs6000-builtins.def (__builtin_saturate_subtract32): New
	built-in function for -mcpu=future.
	(__builtin_saturate_subtract64): Likewise.
	* config/rs6000/rs6000-gen-builtins.cc (enum bif_stanza): Add stanzas
	for -mcpu=future built-ins.
	(stanza_map): Likewise.
	(enable_string): Likewise.
	(struct attrinfo): Likewise.
	(parse_bif_attrs): Likewise.
	(write_decls): Likewise.
	* config/rs6000/rs6000.md (sat_sub<mode>3): Add saturating subtract
	built-in insn declarations.
	(sat_sub<mode>3_dot): Likewise.
	(sat_sub<mode>3_dot2): Likewise.
	* doc/extend.texi (Future PowerPC built-ins): New section.

gcc/testsuite/

	* gcc.target/powerpc/subfus-1.c: New test.
	* gcc.target/powerpc/subfus-2.c: Likewise.
	* lib/target-supports.exp (check_effective_target_powerpc_future_ok):
	New effective target.