We changed to `llvm.compiler.used` because of the behaviour of `gold`,
which refuses to coalesce sections that have different `SHF_GNU_RETAIN`
flags, which causes problems with metadata.
Originally I thought we were going to have to generate two sections
with distinct names and have the runtime look for both of them, but
it turns out that the runtime only wants to see sections that have
`SHF_GNU_RETAIN` in any case. It's really the reflection code that
is interested in being able to see non-retained sections. The upshot
is that we don't need to use `llvm.compiler.used`; it's just fine if
we have duplicate sections, as long as the reflection code looks for
them when it's inspecting an ELF image.
This also means we no longer need to pass `-z nostart-stop-gc` to the
linker if we're using `lld`.
rdar://123504095
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.
