build: build produces a broken object file for iOS

This gets `zig build -Dtarget=aarch64-ios` working. By "working" I mean
it produces an object file without compiler errors. However, the object
file certainly isn't useful since it uses a number of features that will
not work in the iOS sandbox.

This is just an experiment more than anything to see how hard it would be to
get libghostty working within iOS to render a terminal. Note iOS doesn't
support ptys so this wouldn't be a true on-device terminal. The
challenge right now is to just get a terminal rendering (not usable).