Fix various issues with Chinese locales:

• Change short weekday names to use only one Chinese character. (note: this is a somewhat misunfortunate compromise due to the fact that some applications are using short buffer for weekday names, and in ~1905 when 星期 system was created to replace the traditional 七曜 system, which can use 日月火水木金土 to represent Sunday through Saturday with just one character without any confusion).
• for zh_CN locales: use Arabic numerals for month names, matching the practice of all other CJK locales
• Regenerate zh_CN.{GB2312,GBK} locales from zh_CN.UTF-8.

Reported by: ygy
Reviewed by: kevlo
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D18689