Added support for an EFI X64 ABI compatible UnixPkg. With an internal only compiler...

Added support for an EFI X64 ABI compatible UnixPkg. With an internal only compiler I've been able to run checked in X64 EFI shell binary! We are hoping to get the open source LLVM compiler working for this... Since the SEC has to be UNIX ABI to make the POSIX calls it is compiled using a different compiler and the rest of the UnixPkg is compiled with UNIXPKG tool. You just need to point UNIXPKG at your EFI X64 ABI compiler of choice, it should work like MYTOOLS. Some one may want to port this to Linux at some point. To build cd into UnixPkg and ./build64.sh

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10806 6f19259b-4bc3-4df7-8a09-765794883524