Emscripten (wasm32-unknown-emscripten) support

Adds wasm32-unknown-emscripten as a target for nix, plus a CI job that runs the
full test suite under Node.

Source changes to compile on emscripten (which behaves like a Linux/musl libc):

* ioctl: use the Linux ioctl module and a c_int ioctl_num_type on emscripten
  (src/sys/ioctl, src/sys/mod.rs).
* resource: enable getrlimit/setrlimit and the Resource enum via libc::rlimit.
* socket: include emscripten in the SockaddrStorage AF_UNIX (pathname/unnamed)
  decode arms.
* unistd: sethostname is unavailable on emscripten, so gate it out.

The suite spawns OS threads (signal and socket-peer tests), so std is rebuilt
with atomics via -Zbuild-std and linked -pthread with -sPROXY_TO_PTHREAD so the
main thread can block; JSPI (-sJSPI) provides the return-to-host suspension for
blocking reads/writes, and NODERAWFS/NODERAWSOCKETS back the filesystem and
sockets with node's. This needs nightly + rust-src and a JSPI-capable Node
(26+, or 22 with --experimental-wasm-jspi). No custom target spec is required:
nightly now emits the __main_argc_argv entry point (rust-lang/rust#158937).

Genuinely-unsupported operations are ignored with
fork/wait, pty/termios, mkfifo, getpwnam/setgroups/acct, flock, if_nametoindex,
pipe2 FD_CLOEXEC, AF_UNIX SCM_RIGHTS/datagram, thread signal masking, and
setrlimit; a few tests are cfg'd out where the feature is entirely absent
(raw signal actions, socketpair) or the host differs (linkat symlink follow).

Temporary, until the dependencies land upstream:

* Cargo.toml patches libc to guybedford/libc#libc-0.2-emscripten for the
  emscripten externs (rust-lang/libc#5270).
* the CI job builds against the guybedford/emscripten `cf` fork, which carries
  the NODEFS/socket/blocking patches this target depends on
  (emscripten-core/emscripten#27305, #27306).

Suite result: 148 passed, 0 failed, 35 ignored under Node - green on Linux CI.

Author: guybedford

.github/workflows/ci.yml

@@ -676,9 +676,12 @@ jobs:
       - name: test
         run: |
           source ./emsdk/emsdk_env.sh
+          export EM_CONFIG="$EMSDK/.emscripten"
           CARGO_TARGET_WASM32_UNKNOWN_EMSCRIPTEN_RUNNER="$JSPI_NODE" \
           RUSTFLAGS="-A linker_messages -C target-feature=+atomics,+bulk-memory,+mutable-globals -C link-arg=-pthread -C link-arg=-sPROXY_TO_PTHREAD -C link-arg=-sNODERAWSOCKETS -C link-arg=-sNODERAWFS -C link-arg=-sALLOW_MEMORY_GROWTH=1 -C link-arg=-sEXIT_RUNTIME=1 -C link-arg=-sJSPI" \
-          cargo +nightly test --target wasm32-unknown-emscripten -Zbuild-std=std,panic_abort --test test
+          cargo +nightly test --target wasm32-unknown-emscripten -Zbuild-std=std,panic_abort --test test \
+            --config 'patch.crates-io.libc.git="https://github.com/guybedford/libc"' \
+            --config 'patch.crates-io.libc.branch="libc-0.2-emscripten"'
 
       - name: before_cache_script
         run: rm -rf $CARGO_HOME/registry/index

Cargo.toml

@@ -26,9 +26,6 @@ targets = [
   "x86_64-unknown-illumos"
 ]
 
-[patch.crates-io]
-libc = { git = "https://github.com/guybedford/libc", branch = "libc-0.2-emscripten" }
-
 [dependencies]
 libc = { version = "0.2.186", features = ["extra_traits"] }
 bitflags = "2.3.3"