Bump maccore.

New commits in xamarin/maccore:

* xamarin/maccore@50605a5e7f [Xamarin.Hosting] Don't copy single-arch executable over a fat executable. (xamarin#2422)
* xamarin/maccore@9a32b62193 [Xamarin.Hosting] Xcode seems to have changed some logic with regards to getting the primary instruments server. (xamarin#2421)

Diff: https://github.com/xamarin/maccore/compare/3ae9018b640cce7f030eb2fe203a2e66a61e3e04..50605a5e7fc46656d204ef938095c51e3c98ce99

mk/xamarin.mk

@@ -7,7 +7,7 @@ MONO_BRANCH    := $(shell cd $(MONO_PATH) 2> /dev/null && git symbolic-ref --sho
 endif
 
 ifdef ENABLE_XAMARIN
-NEEDED_MACCORE_VERSION := 3ae9018b640cce7f030eb2fe203a2e66a61e3e04
+NEEDED_MACCORE_VERSION := 50605a5e7fc46656d204ef938095c51e3c98ce99
 NEEDED_MACCORE_BRANCH := d16-9
 
 MACCORE_DIRECTORY := maccore