WASI phase 2: WASI arms for perry-runtime platform code #11377

Part of #11375.

Give every platform-specific runtime path a target_os = "wasi" arm so perry-runtime builds for wasm32-wasip2:

• gate perry_sjlj.c build on a WASI sysroot (wasi-sdk) and link libsetjmp
• date.rs timestamp_to_local_components: UTC-only arm (WASI has no TZ database)
• child_process / turnloop_proc: throw a clear "not supported on WASI" error
• dns_resolver.rs / sockets: wasip2 wasi:sockets + ip-name-lookup (socket2 with all already builds for wasip2)
• eh.rs: dladdr / unwind-register code has no wasm equivalent
• event loop: a WASI backend (turnloop is compiled out on wasm)
• perry/thread: sequential parallelMap/parallelFilter, main-thread spawn
• fs: std fs over preopened directories

Done when: cargo check -p perry-runtime --target wasm32-wasip2 --no-default-features is clean and a CI cargo check arm keeps it that way.