gh-96254: Add new wasm build target and support a pkg_config_path

[rchiodo]

In relation to: https://discuss.python.org/t/get-ctypes-working-in-emscripten-build/18466, this adds the following:

• New build target for pthreads support and dynamic linking at the same time
• Ability to set the PKG_CONFIG_PATH so that submodules can be built too

The PKG_CONFIG_PATH addition seems kinda hacky though. @tiran had suggested another workaround to get this to work. I thought I'd put this out there to see what people thought. It is more explicit than having to find where emscripten has its sysroot though.

• Issue: Support a pthread and dynamic link build for wasm #96254
• Issue: Support custom PKG_CONFIG_PATH in the wasm build #96253

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[rchiodo]

/cc @brettcannon @tiran

[bedevere-bot]

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[tiran]

• We don't need the pkg-config workaround. As explained in the ticket, Emscripten has an undocumented EM_PKG_CONFIG_PATH env var. I have a docs PR pending.
• gh-95853: Improve WASM build script (GH-96389) #96389 adds "hidden" experimental build targets.