Clang does not accept `-x objective-c` with WebAssembly target and it
crashes with "Objective-C support is unimplemented for object file
format" for now. `-enable-objc-interop` can work without the objc
runtime support (which is indicated by `objc_interop` feature), so this
adds a new `objc_codegen` feature to require Objective-C support only
at compile-time.