Auto merge of #47730 - alexcrichton:multitrans, r=Mark-Simulacrum

Auto merge of #47730 - alexcrichton:multitrans, r=Mark-Simulacrum

rustc: Split Emscripten to a separate codegen backend

This commit introduces a separately compiled backend for Emscripten, avoiding
compiling the `JSBackend` target in the main LLVM codegen backend. This builds
on the foundation provided by #47671 to create a new codegen backend dedicated
solely to Emscripten, removing the `JSBackend` of the main codegen backend in
the process.

A new field was added to each target for this commit which specifies the backend
to use for translation, the default being `llvm` which is the main backend that
we use. The Emscripten targets specify an `emscripten` backend instead of the
main `llvm` one.

There's a whole bunch of consequences of this change, but I'll try to enumerate
them here:

* A *second* LLVM submodule was added in this commit. The main LLVM submodule
  will soon start to drift from the Emscripten submodule, but currently they're
  both at the same revision.
* Logic was added to rustbuild to *not* build the Emscripten backend by default.
  This is gated behind a `--enable-emscripten` flag to the configure script. By
  default users should neither check out the emscripten submodule nor compile
  it.
* The `init_repo.sh` script was updated to fetch the Emscripten submodule from
  GitHub the same way we do the main LLVM submodule (a tarball fetch).
* The Emscripten backend, turned off by default, is still turned on for a number
  of targets on CI. We'll only be shipping an Emscripten backend with Tier 1
  platforms, though. All cross-compiled platforms will not be receiving an
  Emscripten backend yet.

This commit means that when you download the `rustc` package in Rustup for Tier
1 platforms you'll be receiving two trans backends, one for Emscripten and one
that's the general LLVM backend. If you never compile for Emscripten you'll
never use the Emscripten backend, so we may update this one day to only download
the Emscripten backend when you add the Emscripten target. For now though it's
just an extra 10MB gzip'd.

Closes #46819