OpenMP: Document ompx warnings + add Fortran omx warning [PR106670]

OpenMP: Document ompx warnings + add Fortran omx warning [PR106670]

omp/ompx sentinels are for vendor extensions; as they might be required for
the correctness of the program, a warning should be printable. This patch
documents in the OpenMP 5.2 table the existing warnings, including the new
warning for for fixed source form Fortran.

	PR fortran/106670

gcc/fortran/ChangeLog:

	* scanner.cc (skip_fixed_omp_sentinel): Add -Wsurprising warning
	for 'omx' sentinels with -fopenmp.
	* invoke.texi (-Wsurprising): Document additional warning case.

libgomp/ChangeLog:

	* libgomp.texi (OpenMP 5.2): Add comment to ompx/omx entry.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/ompx-1.c: New test.
	* c-c++-common/gomp/ompx-2.c: New test.
	* g++.dg/gomp/ompx-attrs-1.C: New test.
	* gfortran.dg/gomp/ompx-1.f90: New test.
	* gfortran.dg/gomp/omx-1.f: New test.
	* gfortran.dg/gomp/omx-2.f: New test.