build: make cross-compiling to Windows work on Linux
build: make cross-compiling to Windows work on Linux

Disable the manifest bundling on Windows when cross-compiling on
not-Windows.  With this, it is possible to execute the link command from
CMake which will use cmake to invoke the manifest tool to generate a
manifest and pass that to the linker.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@290836 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx