Currently ReaderLock/Unlock tries CAS only once.
Even if there is moderate contention from other readers only,
ReaderLock/Unlock go onto slow path, which does lots of additional work
before retrying the CAS (since there are only readers, the slow path
logic is not really needed for anything).
Retry CAS while there are only readers.

name                                old cpu/op   new cpu/op   delta
BM_ReaderLock/real_time/threads:1   17.9ns ± 0%  17.9ns ± 0%     ~     (p=0.071 n=5+5)
BM_ReaderLock/real_time/threads:72  11.4µs ± 3%   8.4µs ± 4%  -26.24%  (p=0.008 n=5+5)

PiperOrigin-RevId: 566981511
Change-Id: I432a3c1d85b84943d0ad4776a34fa5bfcf5b3b8e