Print lwlock stats also for aux processes, when built with LWLOCK_STATS

Print lwlock stats also for aux processes, when built with LWLOCK_STATS

InitAuxiliaryProcess() closely resembles InitProcess(), but it didn't
call InitLWLockAccess(). But because InitLWLockAccess() is a no-op
unless compiled with LWLOCK_STATS, and everything works even if it's
not called, the only consequence was that the stats were not printed
for aux processes.

This was an oversight in commit 1c6821be31f, in version 9.5, so it is
missing in all supported branches. But since it only affects
developers using LWLOCK_STATS and no one has complained, no
backpatching.

Discussion: https://www.postgresql.org/message-id/20231130202648[email protected]