[mono-runtimes] Fix bundle usage

Commit 34f437d inadvertently broke mono bundle usage. For example,
the [first commit after the mono bump][xa-01be] *should* have taken
around 3h to build. Instead, it took nearly 7h, because the bundle
wasn't being used, causing the mono runtimes to be rebuilt:

[xa-01be]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/874/

	Target _BuildUnlessCached needs to be built as output file
	'…/xamarin-android//bin/Debug/lib/xamarin.android//../../bcl-tests/System.Runtime.CompilerServices.Unsafe.pdb'
	does not exist.

Indeed, `System.Runtime.CompilerServices.Usafe.pdb` *doesn't* exist,
nor should it, as it's compiled from IL directly.

Fix mono bundle usage by updating `@(_BclTestAssemblyDestination)` to
exclude `System.Runtime.CompilerServices.Unsafe.pdb`. This allows the
`_BuildUnlessCached` target to properly recognize that all required
files *do* exist, thus skipping the mono-runtimes build (as desired).

build-tools/mono-runtimes/mono-runtimes.targets

@@ -58,7 +58,10 @@
     <_BclExcludeDebugSymbols  Include="System.Windows.dll" />
     <_BclExcludeDebugSymbols  Include="System.Xml.Serialization.dll" />
     <_BclTestAssemblyDestination  Include="@(MonoTestAssembly->'$(XAInstallPrefix)\..\..\bcl-tests\%(Identity)')" />
-    <_BclTestAssemblyDestination  Include="@(MonoTestAssembly->'$(XAInstallPrefix)\..\..\bcl-tests\%(Filename).pdb')" />
+    <_BclTestAssemblyDestination
+        Include="@(MonoTestAssembly->'$(XAInstallPrefix)\..\..\bcl-tests\%(Filename).pdb')"
+        Exclude="$(XAInstallPrefix)\..\..\bcl-tests\System.Runtime.CompilerServices.Unsafe.pdb"
+    />
     <_BclTestAssemblyDestination  Include="@(MonoTestRunner->'$(XAInstallPrefix)\..\..\bcl-tests\%(Identity)')" />
     <_BclTestAssemblyDestination  Include="@(MonoTestRunner->'$(XAInstallPrefix)\..\..\bcl-tests\%(Filename).pdb')" />
   </ItemGroup>