Mark FastPathStrongRelationLocks volatile.

Otherwise, the compiler might decide to move modifications to data
within this structure outside the enclosing SpinLockAcquire /
SpinLockRelease pair, leading to shared memory corruption.

This may or may not explain a recent lmgr-related buildfarm failure
on prairiedog, but it needs to be fixed either way.

src/backend/storage/lmgr/lock.c

@@ -238,7 +238,7 @@ typedef struct
 	uint32		count[FAST_PATH_STRONG_LOCK_HASH_PARTITIONS];
 } FastPathStrongRelationLockData;
 
-static FastPathStrongRelationLockData *FastPathStrongRelationLocks;
+static volatile FastPathStrongRelationLockData *FastPathStrongRelationLocks;
 
 
 /*