* Add a variation of wasi-sdk.cmake for pthread-using apps

* Install wasi-sdk-pthread.cmake

* Build wasi-libc with THREAD_MODEL=posix as well

* wasi-sdk-pthread.cmake: target rename to wasm32-wasi-threads

after WebAssembly/wasi-libc#381

* Update wasi-libc to latest HEAD

This pulls in a variety of changes, many of them related to enabling
threads in wasi-libc. There may be more changes necessary before
releasing a prelease version of wasi-sdk that supports the new
`wasm32-wasi-threads` target, but this should get the majority of them.

* review: incorporate WebAssembly/wasi-libc#324

Previously, the release process for wasi-sdk was undocumented and
manual. The `RELEASING.md` document in this commit describes the steps
necessary to publish a new version of `wasi-sdk` as a GitHub release and
provides helpful scripts to automate some of the steps.

With this in place, a future change could add a workflow trigger that
allows running the steps automatically in GitHub actions. Keeping the
steps as scripts in the repository, however, allows developers to re-run
steps themselves in the (likely) case that something goes awry.

Exclude dist-ubuntu-latest to prefer dist-ubuntu-bionic, which is
compatible with wider distributions.
cf.
#273 (comment)
#303

This change switches the MacOS build to target 10.12, an older version
than currently supported. This would close #127. @sbc100 mentions that
this is the setting used by Emscripten and related projects.

This change brings in several helpful PRs (e.g.,
WebAssembly/wasi-libc#397, WebAssembly/wasi-libc#399,
WebAssembly/wasi-libc#401) in anticipation of creating a release based
on LLVM 16.

* llvm-project: update to 16.0.0 release

This changes updates the `src/llvm-project` submodule to the `HEAD` of
`release/16.x`, the same commit used to [release] the LLVM 16.0.0
binaries.

[release]: https://github.com/llvm/llvm-project/releases/tag/llvmorg-16.0.0

* fix: use only Clang's major version in install prefix

Due to [a change] in LLVM, Clang will expect to find the
`libclang_rt.builtins-wasm32.a` file in a path that only contains the
major version (`16`) instead of the entire version (`16.0.0`) as was
previously the case. This change modifies the `CMAKE_INSTALL_PREFIX` to
use Clang's major version only.

[a change]: https://reviews.llvm.org/D125860

* review: only use `llvm_version_major.sh`

Since the `Makefile` can get by with only knowing Clang's major version,
this change removes `llvm_version.sh` and sets `CLANG_VERSION` to use
only the major part.

Sometimes a C codebase might have the odd file written in Assembly. We should force the use of Clang as the assembly compiler for these cases (which it is perfectly capable of doing), and make sure we pass on the WASI compiler triple the same way we do for C and C++.

WebAssembly/wasi-libc#403 fixed an issue with `a_barrier` that should be
included in the next release of wasi-sdk. This change updates wasi-libc
to the latest `HEAD` of `main` to include it.