Moved PlatformDarwin::GetEnvironment() to PlatformPOSIX to fix Linux/llgs issue.

See http://reviews.llvm.org/D5486 for more details.

I was tracking a problem where llgs on linux would not pick up any environment variables. On OSX there is a virtual function PlatformDarwin::GetEnvironment() which correctly sets up the list of environment variables. On linux llgs it defaults to a base class default implementation which clears the list.

I moved the OSX implementation down to PlatformPOSIX. This fixes my problem on linux still works properly on OSX.

Change by Shawn Best.
Slight tweak to convert 'virtual' to 'override' in PlatformDarwin.h virtual method override by Todd.

Tested:
Ubuntu 14.04 x86_64, cmake/ninja build + tests.
MacOSX 10.9.5 x86_64, Xcode 6.1 Beta build + tests.