Define PG_TBLSPC_DIR for path pg_tblspc/ in data folder

Similarly to 2065ddf5e3, this introduces a define for "pg_tblspc".
This makes the style more consistent with the existing PG_STAT_TMP_DIR,
for example.

There is a difference with the other cases with the introduction of
PG_TBLSPC_DIR_SLASH, required in two places for recovery and backups.

Author: Bertrand Drouvot
Reviewed-by: Ashutosh Bapat, Álvaro Herrera, Yugo Nagata, Michael
Paquier
Discussion: https://postgr.es/m/ZryVvjqS9SnV1GPP@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/backup/basebackup.c

@@ -1488,7 +1488,7 @@ sendDir(bbsink *sink, const char *path, int basepathlen, bool sizeonly,
 				if (OidIsValid(spcoid))
 				{
 					relspcoid = spcoid;
-					lookup_path = psprintf("pg_tblspc/%u/%s", spcoid,
+					lookup_path = psprintf("%s/%u/%s", PG_TBLSPC_DIR, spcoid,
 										   tarfilename);
 				}
 				else