Make errno a macro like on other platforms

Currently wasi-libc makes errno a globally visible thread-local variable: https://github.com/WebAssembly/wasi-libc/blob/38f48942fabf1b59e73bca126340b219fa9a78aa/libc-bottom-half/cloudlibc/src/libc/errno/errno.c#L87

This differs from most other platforms - including underlying musl libc https://github.com/WebAssembly/wasi-libc/blob/38f48942fabf1b59e73bca126340b219fa9a78aa/libc-top-half/musl/include/errno.h#L10-L21 - that define errno as just a macro that calls an internal function.

Would it be possible to change this to align the behaviour? I know it's somewhat niche usecase, but it seems to be the only big difference that prevents linking pre-existing object files produced by clang -target wasm32-wasi with Emscripten executables (https://github.com/emscripten-core/emscripten/issues/19236) as object files depend on errno and it's not declared anywhere except wasi-libc.

Start with libc-bottom-half/cloudlibc/src/libc/errno/errno.c and compare it with libc-top-half/musl/include/errno.h. Trace how errno is declared and referenced for the wasm32-wasi target, then verify that pre-existing clang-produced object files can link with Emscripten executables without an undeclared errno dependency.