Improve EBCDIC skip msgs in t/uni

Add more explanation as to why they are skipped
pp_pack.c: Add comment
pp_pack.c: Make pack('U', 0x41) eq 'A'

The 'U' pack/unpack format must be in terms of Unicode code points.
t/test.pl: Fix for non-ASCII platforms
charnames: More fix to work on EBCDIC.

This adds a couple of fixes omitted from
27c3afbd6068ac83b49a11df3e33758ef059027e.