Bump dependencies to the head of their respective branches

Commit list for xamarin/Xamarin.MacDev:

* xamarin/Xamarin.MacDev@39ea45b Add README to point people to xamarin-macios and matching LICENSE (#27)
* xamarin/Xamarin.MacDev@aa495d0 Add ClassKit entitlements key (#26)
* xamarin/Xamarin.MacDev@29e500b Added a public SdkDir property to MonoMacSdk (#25)
* xamarin/Xamarin.MacDev@1a3eeb0 Add NFC entitlement key to EntitlementKeys (#22)
* xamarin/Xamarin.MacDev@22a4d2e Add AllKeys property and GetEntitlementsKeys method (#20)
* xamarin/Xamarin.MacDev@d3931ff Add missing data protection entitlement key (#19)

Diff: 2cff0d7412...39ea45bad4

Commit list for xamarin/maccore:

* xamarin/maccore@76ab6a58ff Bump maciostools to the head of master in preparation for 15.8 branching
* xamarin/maccore@f22f7669d1 [AppleDocReader] Move to use a XamMac app so that we can use the compress lib. (#820)
* xamarin/maccore@33a46996aa Ignore brew failures. (#823)
* xamarin/maccore@758aadc28a Use a custom keychain (xamarin-macios.keychain), and recreate it every time. (#824)

Diff: b9702c7ec8...76ab6a58ff

external/Xamarin.MacDev

@@ -1 +1 @@
-Subproject commit 2cff0d741273341d156de6ce6095808126effd3b
+Subproject commit 39ea45bad4eb85358467b9f316fa6b30aa223e9c

mk/xamarin.mk

@@ -1,5 +1,5 @@
 ifdef ENABLE_XAMARIN
-NEEDED_MACCORE_VERSION := b9702c7ec8fb6dd7dd7972b9bb145c6e82b9ac87
+NEEDED_MACCORE_VERSION := 76ab6a58ff9061d35450f4bc75d075874a8489a8
 NEEDED_MACCORE_BRANCH := master
 
 MACCORE_DIRECTORY := maccore