[SYCL RTC] Make `ToolchainFS` `static thread_local` (#20382)
https://github.com/intel/llvm/pull/19924 essentially made it `static`
but that caused data races that were later fixed by
https://github.com/intel/llvm/pull/20360 changing each use of it to
re-create this in-memory FS (essentially, "removing" `static`),
incurring significant performance costs.
This PR addresses the issue by "adding" `thread_local` instead of
"removing" `static` allowing us to have both no crashes due to data
races and minimal overhead.
No tests added as the one from https://github.com/intel/llvm/pull/20360
is verifying this.
