provide thread name to OS for Solarish systems
Fixes#62302
Passes a Linux bootstrap build. python x.py test src/tools/tidy happy.
I tested this with a small test binary that spawns a few threads, and verified
that:
- on an illumos system lacking the libc function, the binary runs but no OS-level
thread names are set
- on an illumos system with the feature, the binary runs, and the thread names are
visible and correct under tools like MDB, pstack, core dump, etc.