Fix thread naming on Linux, which limits names to 15 bytes.

- In `SetCurrentThreadName`, when on Linux, truncate to 15 bytes, as (at
  least on glibc) `pthread_set_name_np` will otherwise return `ERANGE` and
  do nothing.
- Also, add logging in case `pthread_set_name_np` returns an error
  anyway.  This is Linux-specific, as the Apple and BSD versions of
  `pthread_set_name_np return `void`.
- Change the name for CPU threads in multi-core mode from
  "yuzu:CoreCPUThread_N" (19 bytes) to "yuzu:CPUCore_N" (14 bytes) so it
  fits into the Linux limit.  Some other thread names are also cut off,
  but I didn't bother addressing them as you can guess them from the
  truncated versions.  For a CPU thread, truncation means you can't see
  which core it is!