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.

This is a minimal cherry pick of just the required locale changes.

(cherry picked from commit 466afec3)