Fix cmake build without native target selected.

The cmake (+ MSVC) build is broken if you don't select your native
target.

e.g. 'cmake -D LLVM_TARGETS_TO_BUILD="MyNonNativeTarget" .'

This is because cmake currently sets the LLVM_NATIVE_* definitions
regardless of whether the native target is selected (causing build
errors).

Patch by Mike Gist!

llvm-svn: 112946