en_US: define date_fmt (bug 24046)

The en_US locale use a 12h am/pm format in both d_fmt and d_t_fmt, which
is correct, but does not define date_fmt. This causes the default value
to be used, which is in 24h format.

This patch adds the date_fmt entry to the en_US locale with the same
value as d_t_fmt as the latter already includes the timezone.

Changelog
	[BZ #24046]
	* localedata/locales/en_US (date_fmt): Add, set to
	"%a %d %b %Y %r %Z".