Make init_spin_delay() C89 compliant #2.

Make init_spin_delay() C89 compliant #2.

My previous attempt at doing so, in 80abbeba23, was not sufficient. While that
fixed the problem for bufmgr.c and lwlock.c , s_lock.c still has non-constant
expressions in the struct initializer, because the file/line/function
information comes from the caller of s_lock().

Give up on using a macro, and use a static inline instead.

Discussion: 4369.1460435533@sss.pgh.pa.us