libgomp testsuite: Generalize 'lang_library_path' into a list of 'lang_library_paths'

libgomp testsuite: Generalize 'lang_library_path' into a list of 'lang_library_paths'

..., and use that for libquadmath, too.

	libgomp/
	* testsuite/lib/libgomp.exp (libgomp_target_compile): Generalize
	'lang_library_path' into a list of 'lang_library_paths'.
	* testsuite/libgomp.c++/c++.exp: Adjust.
	* testsuite/libgomp.oacc-c++/c++.exp: Likewise.
	* testsuite/libgomp.fortran/fortran.exp: Adjust.  Use that for
	libquadmath, too.
	* testsuite/libgomp.oacc-fortran/fortran.exp: Likewise.