Support building `vmfirmwareigvm_dll` on non-Windows platforms

With the currently documented set of required dependencies + implemented flowey build logic, the vmfirmwareigvm_dll can only be compiled on Windows platforms.

It seems likely that, given the right set of dependencies / environment variables, it should be possible to get it compiling on other platforms (notably, via WSL2).

Alternatively, Roman suggested rewriting vmfirmwareigvm_dll to instead be a standalone tool which simply crafts a DLL file "out of thin air", using something like the object crate to build a PE executable from scratch. See https://github.com/microsoft/openvmm/pull/291#discussion_r1840799986

This is certainly an option as well, though it comes with its own set of tradeoffs and complexity spirals.

Notably: we would need to "precompile" the resources.rc file + patch it during the build (unless we wanted to try and rewrite a subset of rc.exe functionality, which seems... unwise). EDIT: Roman pointed out that x86_64-w64-mingw32-windres exists for this purpose.

Start by reading the documented dependencies and the existing flowey build logic for vmfirmwareigvm_dll. Compare the non-Windows dependency/environment approach with rewriting the tool using the object crate and x86_64-w64-mingw32-windres, then define one approach whose done condition is successful compilation on a non-Windows platform.