libgomp testsuite: Have each '*.exp' file specify the compiler to use [PR91884]

..., which is still 'GCC_UNDER_TEST' for all of them; no change in behavior.

	PR testsuite/91884
	libgomp/
	* testsuite/lib/libgomp.exp (libgomp_target_compile): Don't
	specify compiler.
	* testsuite/libgomp.c++/c++.exp (ALWAYS_CFLAGS): Specify compiler.
	* testsuite/libgomp.c/c.exp (ALWAYS_CFLAGS): Likewise.
	* testsuite/libgomp.fortran/fortran.exp (ALWAYS_CFLAGS): Likewise.
	* testsuite/libgomp.graphite/graphite.exp (ALWAYS_CFLAGS):
	Likewise.
	* testsuite/libgomp.oacc-c++/c++.exp (ALWAYS_CFLAGS): Likewise.
	* testsuite/libgomp.oacc-c/c.exp (ALWAYS_CFLAGS): Likewise.
	* testsuite/libgomp.oacc-fortran/fortran.exp (ALWAYS_CFLAGS):
	Likewise.