Bump to mono/2017-10/56195c0c; remove corlib test linker (#999)

Context: 5a5ab38c9e

Add tests for the `*Attributes` which were being linked away by mono.
We should be able to remove the linker exclusions now.

external/mono

@@ -1 +1 @@
-Subproject commit 9f86587ade83efdad9901a02073131f12d7de50f
+Subproject commit 56195c0cb36c6f13d8c036a17d47e0267de9e8c4

tests/Xamarin.Android.Bcl-Tests/Resources/LinkerDescription.xml

@@ -4,59 +4,6 @@
     <type fullname="System.Security.Permissions.UIPermission">
       <method name="System.Security.Permissions.IBuiltInPermission.GetTokenIndex"/>
     </type>
-    <type fullname="System.Reflection.AssemblyProductAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyCopyrightAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyTrademarkAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyVersionAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyFileVersionAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyInformationalVersionAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyCultureAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyAlgorithmIdAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyFlagsAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyDelaySignAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyKeyNameAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Reflection.AssemblyKeyFileAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.AttributeUsageAttribute">
-      <method name=".ctor"/>
-      <method name="get_AllowMultiple"/>
-      <method name="set_AllowMultiple"/>
-    </type>
-    <type fullname="System.Runtime.InteropServices.MarshalAsAttribute">
-      <method name=".ctor"/>
-      <field signature="System.Int32 SizeConst"/>
-      <field signature="System.Type MarshalTypeRef"/>
-      <field signature="System.String MarshalCookie"/>
-    </type>
-    <type fullname="System.Runtime.CompilerServices.MethodImplAttribute">
-      <method name=".ctor"/>
-    </type>
-    <type fullname="System.Runtime.InteropServices.DllImportAttribute">
-      <method name=".ctor"/>
-    </type>
     <type fullname="System.Runtime.Remoting.Activation.ActivationServices">
       <method name="CreateProxyForType"/>
     </type>