pg_stat_statements doesn't work with --disable-spinlocks

pg_stat_statements (re-)initializes spinlocks as part of normal
operation. With --disable-spinlock, spinlocks are implemented using
semaphores, and semaphores are expected to be created at postmaster
startup. Hence, you get an assertion failure:

postgres=# select * from pg_stat_statements ;
The connection to the server was lost. Attempting reset: Failed.
!>

TRAP: FailedAssertion("!(!IsUnderPostmaster)", File: "pg_sema.c", Line: 326)

Even if that worked, re-initializing a spinlock with SpinLockInit, like
pg_stat_statement does, would always allocate a new semaphore, so you
would run out very quickly.

- Heikki