Making it easier to build a Halide Runtime DLL

[shoaibkamil]

AFAICT, right now we don't have an easy way to build a standalone Halide runtime DLL on Windows. MSVC does not seem to have an option to export all symbols from an object file, and requires a module definition file that we could programmatically produce.

[alexreinking]

This should be do-able in CMake using the WINDOWS_EXPORT_ALL_SYMBOLS target property and the object file output of the generator (see also #5169). When this is enabled, CMake scans the object files and produces a module definition file as an intermediate target.

[shoaibkamil]

We can make it easier fo CMake users as well, but unfortunately many people are stuck using other build systems. It seems (to my naive understanding) that we could iterate through all the symbols in the runtime module and output the corresponding DEF file; this way it would be another kind of generator output (along with static_library, c_header, etc).