net.mbedtls: enable MBEDTLS_THREADING_C on Windows and macOS (currently Linux/BSD only)

[quaesitor-scientiam]

Summary

In V's vendored mbedtls, MBEDTLS_THREADING_C (mbedtls's internal locking) is enabled
only on Linux/FreeBSD/OpenBSD and #undef'd on Windows and macOS. This means
mbedtls does not mutex-protect its shared state (the ctr_drbg RNG, RSA key blinding,
and the library's global mutexes) on Windows/macOS, so any design that shares an mbedtls
object across threads is unsafe there.

Where it comes from

thirdparty/mbedtls/mbedtls.patch (V's local patch over mbedtls 3.6.x) contains:

#if ( defined(__linux__) || defined(__FreeBSD__) ) || defined (__OpenBSD__)
#define MBEDTLS_THREADING_PTHREAD
#define MBEDTLS_THREADING_C
#else
#undef MBEDTLS_THREADING_PTHREAD
#undef MBEDTLS_THREADING_C
#endif

mbedtls offers two locking backends: MBEDTLS_THREADING_PTHREAD (built-in, needs
pthread.h) and MBEDTLS_THREADING_ALT (the embedder supplies the mutex callbacks via
mbedtls_threading_set_alt() and defines mbedtls_threading_mutex_t in a
threading_alt.h). The patch enables the pthread backend only where pthreads are present
and disables threading otherwise. Windows (MSVC) has no pthread.h and no THREADING_ALT
shim was written; macOS has pthreads but is simply not listed in the gate (apparent
oversight).

Why it matters

Today it is harmless: V never shares mutable mbedtls state across threads — client
SSLConns each own their config + RNG, and the TLS server handshakes run serially on the
accept thread. But it blocks any future concurrency that shares an mbedtls config/RNG
across threads, in particular parallelizing TLS server handshakes across worker threads
(the deferred item #3 from #27433). On Windows/macOS that would be a data race on the
shared ctr_drbg and RSA key state.

Proposed fix

• macOS: add || defined(__APPLE__) to the gate — macOS has pthreads, so
  MBEDTLS_THREADING_PTHREAD works as-is.
• Windows: implement MBEDTLS_THREADING_ALT over Win32 CRITICAL_SECTION:
  1. threading_alt.h defining mbedtls_threading_mutex_t (a CRITICAL_SECTION + valid
     flag);
  2. four C callbacks wrapping InitializeCriticalSection/EnterCriticalSection/
     LeaveCriticalSection/DeleteCriticalSection;
  3. define MBEDTLS_THREADING_C + MBEDTLS_THREADING_ALT on _WIN32;
  4. call mbedtls_threading_set_alt(...) once at startup (V module init()), before any
     TLS use;
  5. rebuild the mbedtls objects.
• Fold the change into mbedtls.patch so it survives the next mbedtls upgrade.

Caveats

• Enables (does not require) cross-thread sharing; small per-lock overhead once on.
• set_alt must run before mbedtls touches any global mutex (module-init() handles this).
• Touches thirdparty/ and the prebuilt mbedtls objects (rebuild needed).

Plan

Implementing on a branch; macOS (pthread) + Windows (THREADING_ALT). macOS will be tested
on Apple Silicon (M-series). Prerequisite for safe parallel TLS handshakes (#27433 item #3).

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