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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161685 91177308-0d34-0410-b5e6-96231b3b80d8