crt: Use the right options in the UCRT non-inline _snwprintf

In the inline form of _snwprintf, we set
_CRT_INTERNAL_PRINTF_LEGACY_VSPRINTF_NULL_TERMINATION,
but in the non-inline version, we just called
vsnwprintf, which doesn't set that option.

Instead call __stdio_common_vswprintf directly, with the right
options.

The previous form stemmed from the original UCRT implementation
in bc6a87488995675ae80c312e3585cb1ace739b43.

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