Tidy up Mutex::[Reader]TryLock codegen by outlining slow path
and non-tail function call, and un-unrolling the loop.

Current codegen:
https://gist.githubusercontent.com/dvyukov/a4d353fd71ac873af9332c1340675b60/raw/226537ffa305b25a79ef3a85277fa870fee5191d/gistfile1.txt

New codegen:
https://gist.githubusercontent.com/dvyukov/686a094c5aa357025689764f155e5a29/raw/e3125c1cdb5669fac60faf336e2f60395e29d888/gistfile1.txt

name                                   old cpu/op   new cpu/op   delta
BM_TryLock                             18.0ns ± 0%  17.7ns ± 0%   -1.64%  (p=0.016 n=4+5)
BM_ReaderTryLock/real_time/threads:1   17.9ns ± 0%  17.9ns ± 0%   -0.10%  (p=0.016 n=5+5)
BM_ReaderTryLock/real_time/threads:72  9.61µs ± 8%  8.42µs ± 7%  -12.37%  (p=0.008 n=5+5)

PiperOrigin-RevId: 567006472
Change-Id: Iea0747e71bbf2dc1f00c70a4235203071d795b99