Split WaitEventSet functions to separate source file

latch.c now only contains the Latch related functions, which build on
the WaitEventSet abstraction. Most of the platform-dependent stuff is
now in waiteventset.c.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/8a507fb6-df28-49d3-81a5-ede180d7f0fb@iki.fi

src/backend/storage/ipc/Makefile

@@ -25,6 +25,7 @@ OBJS = \
 	signalfuncs.o \
 	sinval.o \
 	sinvaladt.o \
-	standby.o
+	standby.o \
+	waiteventset.o
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/storage/ipc/meson.build

@@ -18,5 +18,6 @@ backend_sources += files(
   'sinval.c',
   'sinvaladt.c',
   'standby.c',
+  'waiteventset.c',
 
 )