x86: Fix TEST_NAME to make it a string in tst-strncmp-rtm.c

Previously TEST_NAME was passing a function pointer. This didn't fail
because of the -Wno-error flag (to allow for overflow sizes passed
to strncmp/wcsncmp)

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
(cherry picked from commit b98d0bbf)