headers: Don't use __mingw_wcstod/wcstof on UCRT

The UCRT provided versions should be fully C99 compliant.

This matches what was done for strtod/strtof for the initial
configuration for UCRT in 265d0a5e3f087ed6bd68412cceed5e27e3ef62ae.

Also clarify the endif comment, and make the condition in wchar.h
consistent with the one in stdlib.h.

Signed-off-by: Martin Storsjö <martin@martin.st>