[AArch64] Don't define __LP64__ when targeting Windows

Windows/ARM64 is a LLP64 environment, so don't set this default
define.

Differential Revision: https://reviews.llvm.org/D36098

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309619 91177308-0d34-0410-b5e6-96231b3b80d8