Improve NaN payload testing

Improve NaN payload testing

There are two separate sets of tests of NaN payloads in glibc:

* libm-test-{get,set}payload* verify that getpayload, setpayload,
  setpayloadsig and __builtin_nan functions are consistent in their
  payload handling.

* test-nan-payload verifies that strtod-family functions and the
  not-built-in nan functions are consistent in their payload handling.

Nothing, however, connects the two sets of functions (i.e., verifies
that strtod / nan are consistent with getpayload / setpayload /
__builtin_nan).

Improve test-nan-payload to check actual payload value with getpayload
rather than just verifying that the strtod and nan functions produce
the same NaN.  Also check that the NaNs produced aren't signaling and
extend the tests to cover _FloatN / _FloatNx.

Tested for x86_64.