Temporarily give System.Runtime.Extensions its own Environment.* copy (dotnet/corefx...

Temporarily give System.Runtime.Extensions its own Environment.* copy (dotnet/corefx#34951)

The files were moved to shared to propagate them to coreclr.  But changes being made in coreclr are going to break System.Runtime.Extensions when the files mirror back.  As a temporary stop-gap, I'm creating local copies of the files and taking System.Runtime.Environment off of the shared copies.  Then once coreclr builds with the updates and corefx consumes the updated coreclr, we'll just delete these temporary copies and remove the relevant entries entirely from System.Runtime.Extensions.

Commit migrated from https://github.com/dotnet/corefx/commit/2f6536eb0a2f6ac246f99382d0235dffd75eb1c4