Draft: Wasm backend Template Haskell support

Closes #24376. WIP, not working yet.

TODOs:

• host_fully_static logic in hadrian/ci
• Fix gnu libtool support for shared library feature detection on wasm32-wasi target
• Build shared library for libffi-wasm
• Build shared library for in-tree GMP
• PIC and shared libraries support for unreg backend
• PIC and shared libraries support for NCG
• dlopen implementation
• External interpreter logic
• JSFFI support in dynamic code
• Fix testsuite
• Off-CI manual testing with real world TH splices
• Source notes, user manual, release notes, etc