-shared: auto-call GC_allow_register_threads() from a library constructor

[eptx]

Reproduced on V 0.5.1 (commit 99f141f in upstream master);
verified no relevant commits land between that and current
master b6dfae5. OS: macOS 26.4.1 ARM64.

Problem

A V -shared library built with the default Boehm GC is unusable from
host-spawned threads (Rust cargo workers, C# tasks, Java JNI threads)
because libgc aborts with "Collecting from unknown thread" whenever a
collection is triggered from a thread libgc has not been told about.

To register host threads, the binding must:

1. Call GC_allow_register_threads() once at module load (process-
   level).
2. Call GC_register_my_thread(&stack_base) per thread before that
   thread enters any V code.

Step 2 is inherently per-binding (the binding knows when it spawns a
thread). Step 1 is process-level and could be done by V itself.

Proposed fix

When emitting a -shared library and the default GC is Boehm, emit a
constructor that runs at dlopen time:

__attribute__((constructor))
static void _v_shared_lib_init(void) {
    GC_allow_register_threads();
}

That is the entire change. After it, host bindings only need the
per-thread call, which they need anyway.

Downstream evidence

CX currently exports its own cx_init symbol whose entire body is

void cx_gc_allow_register_threads(void) {
    GC_allow_register_threads();
}

Every binding (Rust, C#, Java, …) is required to call cx_init() at
module load, and every binding's onboarding doc has a "remember to
call cx_init" footgun. With this V change, the cx_init export goes
away.

Why not GC-side?

bdwgc deliberately requires the host to opt in to thread-registration,
because for single-threaded users the cost of the lookup tables is
real. V already knows when it's building a multi-threaded -shared
library; emitting the constructor is the right layer.

Note

You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.