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.
Avoid a temporary file and executing FileCheck multiple types and prefer
multiple check prefixes and streaming. Additionally, enable some of
previously XFAIL'ed tests on Linux as well as tests that were marked as
requiring Objective-C interop.
This prevents the linker from trying to emit relative relocations to locally-defined public symbols into dynamic libraries, which gives ld.so heartache.
