deps: uv: restore prior signal disposition instead of SIG_DFL (DCP-4748)

[philippe-distributive]

Summary

When a Node environment is torn down inside an Android app process
(node::FreeEnvironment() → uv_close() on a UV_SIGNAL handle), libuv
resets the signal's disposition to SIG_DFL. On Android that sigaction
call is intercepted by ART's libsigchain, which logs an ERROR-level
stack trace on every worker stop:

  E libsigchain: Setting SIGSEGV to SIG_DFL
  E libsigchain:   ... uv_close ... node::Environment::CleanupHandles() ...

ART claims SIGSEGV/SIGBUS/SIGFPE/SIGILL/SIGTRAP/SIGABRT for implicit
null-checks, stack-overflow detection, etc., so resetting one to SIG_DFL
is also semantically wrong, not just noisy.

Root cause

uv__signal_unregister_handler() unconditionally installs SIG_DFL once
the last watcher for a signal is removed. libuv never recorded what was
installed before it added its multiplexer — there's a long-standing
/* XXX save old action so we can restore it later on? */ TODO at the
install site to prove it.

Fix

Resolve that TODO. uv__signal_register_handler() now captures the
previous struct sigaction (straight into a per-signum slot) on the
genuine "no libuv handler → installed" transition, gated by a save
flag so the oneshot↔regular re-registration paths never overwrite it
with libuv's own handler. uv__signal_unregister_handler() restores that
saved disposition instead of forcing SIG_DFL, falling back to SIG_DFL

Platform-agnostic and strictly more correct than a blind reset; on Android
the restored handler is ART's (kept by libsigchain), so no SIG_DFL reset
— and no libsigchain report — occurs. Zero hot-path cost (only runs on the
first register / last unregister per signum); the saved table is touched
only under the existing global signal lock.

Alternatives rejected

• Skip the reset under #ifdef __ANDROID__ — Android-specific and
  leaves libuv's multiplexer chained behind ART forever.
• Raw rt_sigaction(SIG_DFL) syscall to bypass libsigchain — unsafe:
  it overwrites libsigchain's kernel-level handler (the one ART chains
  behind), silencing the log by breaking ART's signal handling.

Testing

• libuv signal suite built for the host: 10/10 pass, including
  we_get_signals_mixed (the oneshot↔regular path the save gate
  guards), signal_pending_on_close, signal_close_loop_alive.
• Cross-compiled libnode.so for all four Android ABIs (arm64-v8a,
  x86_64, armeabi-v7a, x86).
• On an x86_64 emulator (API 35): worker start→stop, libsigchain "Setting SIGSEGV to SIG_DFL" count 1 → 0, with the cleanup path
  confirmed exercised (FreeEnvironment runs, libuv loop closed cleanly).

Upstreaming

deps/uv here is byte-identical to upstream libuv v1.51.0, so this is a
clean candidate for an upstream libuv PR (any embedder benefits).

Rollout

Cut a tag (e.g. dcp/3.2.1) → bump node-build's GIT_TAG → rebuild the
Android node_api zips → bump the URL/SHA in
dcp-native/externals/node_api/CMakeLists.txt → drop the rebuilt
libnode.so into android-worker jnilibs/.

[philippe-distributive]

fixed upstream in libuv/libuv#5157