Introduce a UseSystemLibraries option to use system libunwind

UseSystemLibraries lets users control if they want to use the system
versions of libraries or bundled libraries. The only use case where this
applies currently is coreclr's libunwind.

dir.props

@@ -8,6 +8,7 @@
     <_IsBootstrapping Condition="'$(BootstrapBuildToolsDir)' != ''">true</_IsBootstrapping>
 
     <PortableBuild Condition="'$(PortableBuild)' == ''">false</PortableBuild>
+    <UseSystemLibraries Condition="'$(UseSystemLibraries)' == ''">true</UseSystemLibraries>
   </PropertyGroup>
 
   <PropertyGroup>

repos/coreclr.proj

@@ -4,6 +4,7 @@
   <PropertyGroup>
     <BuildArguments>$(Platform) $(Configuration) skiptests -PortableBuild=$(PortableBuild)</BuildArguments>
     <BuildArguments Condition="'$(OS)' != 'Windows_NT'">$(BuildArguments) msbuildonunsupportedplatform</BuildArguments>
+    <BuildArguments Condition="'$(UseSystemLibraries)' != 'false'">$(BuildArguments) cmakeargs -DCLR_CMAKE_USE_SYSTEM_LIBUNWIND=TRUE</BuildArguments>
     <BuildArguments Condition="$(Platform.Contains('arm'))">$(BuildArguments) skipnuget cross -skiprestore cmakeargs -DFEATURE_GDBJIT=TRUE</BuildArguments>
     <BuildCommand>$(ProjectDirectory)/build$(ShellExtension) $(BuildArguments) --</BuildCommand>
     <BuildCommand Condition="$(Platform.Contains('arm'))">$(ArmEnvironmentVariables) $(BuildCommand)</BuildCommand>