x/text/encoding/charmap: add more OEM codepages

Add support for many of the single-byte Windows OEM code pages listed at
https://msdn.microsoft.com/en-us/goglobal/bb964655.
Download the code page data from the ICU source.

Change Code Page 437 to use the data from ICU too. This changes the
mapping of a few characters, including one that was used in a test.

Fixes golang/go#13043

Change-Id: Ife0e33071195054ba23fd6eca3448aa6a99618f0
Reviewed-on: https://go-review.googlesource.com/16364
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
3 files changed