[NFC][compiler-rt][hwasan] Re-use ring buffer size calculation

[leonardchan]

Summary

Users can call HwasanThreadList::GetRingBufferSize rather than RingBufferSize to prevent having to do the calculation in RingBufferSize. This will be useful for Fuchsia where we plan to initialize the stack ring buffer separately from the rest of thread initialization.

compiler-rt/lib/hwasan/hwasan_thread_list.h

void RemoveThreadStats(Thread *t) {
     stats_.total_stack_size -= t->stack_size();
   }
 
   ThreadStats GetThreadStats() {
     SpinMutexLock l(&stats_mutex_);
     return stats_;
   }
 
+  uptr GetRingBufferSize() const { return ring_buffer_size_; }
+
  private:
   Thread *AllocThread() {
     SpinMutexLock l(&free_space_mutex_);
     uptr align = ring_buffer_size_ * 2;
     CHECK(IsAligned(free_space_, align));
     Thread *t = (Thread *)(free_space_ + ring_buffer_size_);
     free_space_ += thread_alloc_size_;
     CHECK(free_space_ <= free_space_end_ && "out of thread memory");