Fixing date patterns on Linux.

Fixing date patterns on Linux.

1. Getting more LongDatePatterns by getting both full and long ICU patterns.
2. Getting more ShortDatePatterns by getting both medium and short ICU patterns.  Still keeping the "yMd" pattern, since this closely matches what is used on Windows.
3. Removing any duplicates in the date patterns.
4. "Normalizing" the date patterns from ICU format to .NET format.
  a. "EEEE", "eeee" and "cccc" is replaced with "dddd"
  b. "LLLL" is replaced with "MMMM"
  c. "G" is replaced with "g"
  d. A single 'y' is replaced with 'yyyy'

Commit migrated from https://github.com/dotnet/coreclr/commit/9dc6f27b77af7bf4272f63d9af2ec5b5ca2ec43a