This adds support for building WASI shared libraries per
https://github.com/WebAssembly/tool-conventions/blob/main/DynamicLinking.md.

For the time being, the goal is to allow "pseudo-dynamic" linking using the
Component Model per
https://github.com/WebAssembly/component-model/blob/main/design/mvp/examples/SharedEverythingDynamicLinking.md.
This requires all libraries to be available when the component is created, but
still allows runtime symbol resolution via `dlopen`/`dlsym` backed by a static
lookup table.  This is sufficient to support Python native extensions, for
example.  A complete demo using `wit-component` is available at
https://github.com/dicej/component-linking-demo.

This requires https://reviews.llvm.org/D153293, which we will need to backport
to LLVM 16 we're ready to upgrade to LLVM 17, hence the
llvm-D153293-backport.patch file.

Signed-off-by: Joel Dice <[email protected]>

This is where I originally meant the directive to go, but it didn't seem to work
the first time I tried it, hence the added lines in wasi-sdk.cmake and
wasi-sdk-pthread.cmake.  However, it seems to work fine now, so I'm switching it
back to be consistent with other similar CMake flags.

Signed-off-by: Joel Dice <[email protected]>

Signed-off-by: Joel Dice <[email protected]>

As of WebAssembly/wasi-libc#442, these are determined
automatically.

Signed-off-by: Joel Dice <[email protected]>