[Kernel] Add null-safe XThread::TryGetCurrentThread() accessor

The additive event_log extraction tracer needs a current-thread lookup that
returns nullptr instead of asserting when called from boot code before any
XThread exists (GetCurrentThread asserts on non-guest threads). New read-only
static accessor over the thread-local; no behaviour change. Fixes the native
Linux build of the phase-a args/file.read tracer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/xenia/kernel/xthread.cc

@@ -117,6 +117,8 @@ XThread* XThread::GetCurrentThread() {
   return thread;
 }
 
+XThread* XThread::TryGetCurrentThread() { return current_xthread_tls_; }
+
 uint32_t XThread::GetCurrentThreadHandle() {
   XThread* thread = XThread::GetCurrentThread();
   return thread->handle();