linker: implement -build-mode:static on non-windows targets

[laytan]

Should either:

• build an object file that is ready to put in a static library with ar and the likes
• or also actually call ar and put the one object file in it

[ngortheone]

I built odin compiler on FreeBSD today and tried building demo like this:

./odin build examples/demo -build-mode:static
Internal Compiler Error: TODO(bill): -build-mode:static on non-windows targets

illegal hardware instruction (core dumped)  ./odin build examples/demo -build-mode:static

if (build_context.build_mode == BuildMode_StaticLibrary) {
    compiler_error("TODO(bill): -build-mode:static on non-windows targets");
}

Can -build-mode:static be used with binaries or is it for libraries only?