classes/conf: Add eventmasks for event handlers

Now that bitbake supports masking events for event handlers, lets use
this so event handlers are only called for events they care about. This
lets us simplify the code indentation a bit at least as well as mildly
improving the event handling performance.

(From OE-Core rev: bff73743280f9eafebe4591f7368ead91a4eb74d)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/multilib.bbclass

@@ -1,7 +1,4 @@
 python multilib_virtclass_handler () {
-    if not isinstance(e, bb.event.RecipePreFinalise):
-        return
-
     cls = e.data.getVar("BBEXTENDCURR", True)
     variant = e.data.getVar("BBEXTENDVARIANT", True)
     if cls != "multilib" or not variant:
@@ -60,6 +57,7 @@ python multilib_virtclass_handler () {
 }
 
 addhandler multilib_virtclass_handler
+multilib_virtclass_handler[eventmask] = "bb.event.RecipePreFinalise"
 
 STAGINGCC_prepend = "${BBEXTENDVARIANT}-"