Renames x86_64 specific asm files

Renames all x86_64 specific assembly files to consistently
end in _x86_64.asm. This will be useful for build systems to
handle these files differently.
All new 64-bit specific assembly files should use the new
naming convention.

Change-Id: I36c89584967c82ffc4088b1b5044ac15d2bb7536