libgomp testsuite: As appropriate, use the 'gcc', 'g++', 'gfortran' driver [PR91884]

..., that is, 'GCC_UNDER_TEST', 'GXX_UNDER_TEST', 'GFORTRAN_UNDER_TEST' instead
of 'GCC_UNDER_TEST' for all of them.  No need anymore for 'gcc -lstdc++ -x c++'
for C++ code, or 'gcc -lgfortran' plus conditional '-lquadmath' for Fortran
code.  (Getting rid of explicit '-foffload=-lgfortran' is for another day.)

	PR testsuite/91884
	libgomp/
	* configure.ac: 'AC_SUBST(CXX)'.
	* configure: Regenerate.
	* Makefile.in: Likewise.
	* testsuite/Makefile.in: Likewise.
	* testsuite/libgomp-site-extra.exp.in (GXX_UNDER_TEST)
	(GFORTRAN_UNDER_TEST): Set.
	* testsuite/lib/libgomp.exp (libgomp_init): Adjust.
	* testsuite/libgomp.c++/c++.exp: Use 'GXX_UNDER_TEST'.
	* testsuite/libgomp.oacc-c++/c++.exp: Likewise.
	* testsuite/libgomp.fortran/fortran.exp: Use
	'GFORTRAN_UNDER_TEST'.
	* testsuite/libgomp.oacc-fortran/fortran.exp: Likewise.