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

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.

Differential Revision: https://reviews.llvm.org/D104823