Auto merge of #111713 - Zoxc:lock-switch, r=nnethercote

Use conditional synchronization for Lock

This changes `Lock` to use synchronization only if `mode::is_dyn_thread_safe` could be true. This reduces overhead for the parallel compiler running with 1 thread.

The emitters are changed to use `DynSend` instead of `Send` so they can still use `Lock`.

A Rayon thread pool is not used with 1 thread anymore, as session globals contains `Lock`s which are no longer `Sync`.

Performance improvement with 1 thread and `cfg(parallel_compiler)`:
<table><tr><td rowspan="2">Benchmark</td><td colspan="1"><b>Before</b></th><td colspan="2"><b>After</b></th></tr><tr><td align="right">Time</td><td align="right">Time</td><td align="right">%</th></tr><tr><td>🟣 <b>clap</b>:check</td><td align="right">1.7665s</td><td align="right">1.7336s</td><td align="right">💚  -1.86%</td></tr><tr><td>🟣 <b>hyper</b>:check</td><td align="right">0.2780s</td><td align="right">0.2736s</td><td align="right">💚  -1.61%</td></tr><tr><td>🟣 <b>regex</b>:check</td><td align="right">0.9994s</td><td align="right">0.9824s</td><td align="right">💚  -1.70%</td></tr><tr><td>🟣 <b>syn</b>:check</td><td align="right">1.5875s</td><td align="right">1.5656s</td><td align="right">💚  -1.38%</td></tr><tr><td>🟣 <b>syntex_syntax</b>:check</td><td align="right">6.0682s</td><td align="right">5.9532s</td><td align="right">💚  -1.90%</td></tr><tr><td>Total</td><td align="right">10.6997s</td><td align="right">10.5083s</td><td align="right">💚  -1.79%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">0.9831s</td><td align="right">💚  -1.69%</td></tr></table>

cc `@SparrowLii`