rtl: builtins: (not just) rs6000: Add builtins for fegetround, feclearexcept and...

rtl: builtins: (not just) rs6000: Add builtins for fegetround, feclearexcept and feraiseexcept [PR94193]

This optimizations were originally in glibc, but was removed
and suggested that they were a good fit as gcc builtins[1].

feclearexcept and feraiseexcept were extended (in comparison to the
glibc version) to accept any combination of the accepted flags, not
limited to just one flag bit at a time anymore.

The builtin expanders needs knowledge of the target libc's FE_*
values, so they are limited to expand only to suitable libcs.

[1] https://sourceware.org/legacy-ml/libc-alpha/2020-03/msg00047.html
    https://sourceware.org/legacy-ml/libc-alpha/2020-03/msg00080.html

2020-08-13  Raoni Fassina Firmino  <raoni@linux.ibm.com>

gcc/
	PR target/94193
	* builtins.cc (expand_builtin_fegetround): New function.
	(expand_builtin_feclear_feraise_except): New function.
	(expand_builtin): Add cases for BUILT_IN_FEGETROUND,
	BUILT_IN_FECLEAREXCEPT and BUILT_IN_FERAISEEXCEPT.
	* config/rs6000/rs6000.md (fegetroundsi): New pattern.
	(feclearexceptsi): New Pattern.
	(feraiseexceptsi): New Pattern.
	* doc/extend.texi: Add a new introductory paragraph about the
	new builtins.
	* doc/md.texi: (fegetround@var{m}): Document new optab.
	(feclearexcept@var{m}): Document new optab.
	(feraiseexcept@var{m}): Document new optab.
	* optabs.def (fegetround_optab): New optab.
	(feclearexcept_optab): New optab.
	(feraiseexcept_optab): New optab.

gcc/testsuite/
	PR target/94193
	* gcc.target/powerpc/builtin-feclearexcept-feraiseexcept-1.c: New test.
	* gcc.target/powerpc/builtin-feclearexcept-feraiseexcept-2.c: New test.
	* gcc.target/powerpc/builtin-fegetround.c: New test.

Signed-off-by: Raoni Fassina Firmino <raoni@linux.ibm.com>