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: https://llvm.org/svn/llvm-project/libcxx/trunk@290836 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed