[lsan][Darwin][nfc] Fix thread vector size

[lgrey]

Summary

The reserve constructor was removed in 44f55509d75d8c67077810bb6d9f3bedaea05831 but this one was missed.
As a result, we attempt to iterate through 1024 threads each time.

[vitalybuka]

compiler-rt/lib/sanitizer_common/sanitizer_stoptheworld_mac.cpp

32–33

threads_() is NOOP, as it already has default constructor
reserve is not needed as InternalMmapVector will reserve 4k on the first pushback

[lgrey]

Changed to =default

compiler-rt/lib/sanitizer_common/sanitizer_stoptheworld_mac.cpp

 namespace __sanitizer {
 typedef struct {
   tid_t tid;
   thread_t thread;
 } SuspendedThreadInfo;
 
 class SuspendedThreadsListMac final : public SuspendedThreadsList {
  public:
-  SuspendedThreadsListMac() : threads_(1024) {}
+  SuspendedThreadsListMac() = default;
 
   tid_t GetThreadID(uptr index) const override;
   thread_t GetThread(uptr index) const;
   uptr ThreadCount() const override;
   bool ContainsThread(thread_t thread) const;
   void Append(thread_t thread);
 
   PtraceRegistersStatus GetRegistersAndSP(uptr index,
                                           InternalMmapVector<uptr> *buffer,