Correct test descriptors in libm-test-pown.inc

Correct test descriptors in libm-test-pown.inc

While working on implementing compoundn, I noticed that
libm-test-pown.inc was wrongly using TEST_ff_f and AUTO_TESTS_ff_f
when the actual types involved meant fL_f should be used instead of
ff_f; fix to use the correct descriptor strings for pown.  (These
strings affect how gen-libm-test.py generates a C file in some cases.
The structure type test_fL_f_data for expected results and the use of
RUN_TEST_LOOP_fL_f in the ALL_RM_TEST call were already correct.)

Tested for x86_64.  The generated libm-test-pown.c was actually
unchanged, but the old descriptor strings were still logically
incorrect.