Use ALL_RM_TEST for more libm tests.

Continuing the move to using ALL_RM_TEST for tests in libm-test.inc,
this patch converts the tests of fdim, ldexp and scalb.  fdim and
scalb are cases where tests could depend on the rounding mode though
none of the present test inputs do; ldexp is such a case where the
function is equivalent to scalbn (for binary floating point) and the
tests used were a subset of those for scalbn, so this patch makes
ldexp testing use the scalbn tests, as done for other cases of libm
function aliases.

Tested x86_64 and x86.

	* math/libm-test.inc (fdim_test): Use ALL_RM_TEST.
	(ldexp_test_data): Remove.
	(ldexp_test): Move to after scalbn_test.  Use ALL_RM_TEST with
	scalbn_test_data.
	(scalb_test): Use ALL_RM_TEST.