Fix `IPHONEOS_DEPLOYMENT_TARGET` on Mac Catalyst
Some of the target code invalidly assumed that the deployment target variable on Mac Catalyst is `MACOSX_DEPLOYMENT_TARGET`, which is wrong, Mac Catalyst uses the same environment variable as iOS.
Additionally, the deployment target was hardcoded to `14.0`, I've lowered this to `13.1` ([same default as Clang](d022f32c73/clang/lib/Driver/ToolChains/Darwin.cpp (L2038))), and made it properly load from the environment.
This shouldn't require any changes to the `cc` crate, as that uses `rustc --print=deployment-target` to get this information automatically.
CC `@BlackHoleFox`
r? `@rust-lang/macos`
7d61394948