Fixes iOS compiling.

[simlay]

It's unclear if this finishes #29 but this deals with a pretty annoying edge case when building foraarch64-apple-ios as described here: rust-lang/rust-bindgen#1211.

Also, you might notice that rustup target list | grep ios yields something like:

aarch64-apple-ios (installed)
armv7-apple-ios (installed)
armv7s-apple-ios (installed)
i386-apple-ios (installed)
x86_64-apple-ios (installed)

The i386 and x86_64 targets don't have architecture support in the iPhoneOS13.2.sdk it seems. I can share the stack trace if you'd like. For now compiling for aarch64, armv7 and armv7s I think is more than enough.