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.)