[clang][WebAssembly] Support wasm32-wasi shared libraries

This adds support for Emscripten-style shared libraries [1] to
non-emscripten targets, such as `wasm32-wasi`.  Previously, only static
linking was supported, and the `-shared` and `-fPIC` flags were simply
ignored.  Now both flags are honored.

Since WASI runtimes do not necessarily include JavaScript support, we
cannot rely on the JS-based Emscripten linker to link shared libraries.
Instead, we link them using the Component Model proposal [2].

We have prototyped shared library support in `wasi-sdk` [3] and put
together a demo [4] which uses a patched version of `wit-component` [5]
to link libraries using the Component Model.  We plan to submit the
required changes upstream to the respective repositories in the next
week or two.

[1] https://github.com/WebAssembly/tool-conventions/blob/main/DynamicLinking.md
[2] https://github.com/WebAssembly/component-model/blob/main/design/mvp/examples/SharedEverythingDynamicLinking.md
[3] https://github.com/dicej/wasi-sdk/tree/dynamic-linking
[4] https://github.com/dicej/component-linking-demo
[5] https://github.com/bytecodealliance/wasm-tools/tree/main/crates/wit-component

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Reviewed By: sbc100

Differential Revision: https://reviews.llvm.org/D153293

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