ja_JP locale: Add entry for the new Japanese era [BZ #22964]

The Japanese era name will be changed on May 1, 2019.  The Japanese
government made a preliminary announcement on April 1, 2019.

The glibc ja_JP locale must be updated to include the new era name for
strftime's alternative year format support.

Checked on x86_64-linux-gnu.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

ChangeLog:

	[BZ #22964]
	* localedata/locales/ja_JP (LC_TIME): Add entry for the new Japanese
	era.
	* time/tst-strftime2.c (dates): Add 2019-04-30 and 2019-05-01.
	(mkreftable): Add rules for the new Japanese era and the new dates.