Fix cmake build without native target selected.
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!

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@112946 91177308-0d34-0410-b5e6-96231b3b80d8

chubbymaggie/asap,llvm-mirror/llvm,llvm-mirror/llvm,dslab-epfl/asap,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,chubbymaggie/asap,chubbymaggie/asap,dslab-epfl/asap,dslab-epfl/asap,GPUOpen-Drivers/llvm,chubbymaggie/asap,dslab-epfl/asap,GPUOpen-Drivers/llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm