cal: fix first day of the week calculation on BE systems

cal: fix first day of the week calculation on BE systems

This reverts commit dcb54fafb128ab41772ae217afc6a7612e2cc446,
"cal: remove gcc-ism from nl_langinfo() call".

The code:

	int wfd = (int)(intptr_t) nl_langinfo(_NL_TIME_WEEK_1STDAY);

does not work on big-endian machines. The original solution based on
union is better.

Note that the "type punning" is not gcc-ism any more, it's allowed
by C99 (6.5.2.3).

Reported-by: Joseph Jezak <josejx@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>