Frontend: define _LP64 in a target-independent way

Instead of adding it to each individual subclass in
Targets.cpp, simply check the appropriate target
values.

Where before it was only on x86_64 and ppc64, it's now
also defined on mips64 and nvptx64.

Also add a bunch of negative tests to ensure it is *not*
defined on any other architectures while we're here.

llvm-svn: 161685