OvmfPkg LoadLinuxLib: Use kernel's EFI entry point where available

OvmfPkg LoadLinuxLib: Use kernel's EFI entry point where available

Usage of the EFI entry point was made feasible in the kernel
x64 boot protocol 2.12 where a 32-bit & 64-bit entry point
became well defined.

http://git.kernel.org/linus/09c205af

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14132 6f19259b-4bc3-4df7-8a09-765794883524