Enable FEATURE_APPDOMAIN for mono builds
Enable FEATURE_APPDOMAIN for mono builds

FEATURE_APPDOMAIN was disabled for non-coreclr builds targeting mono.
But since app domains is supported on mono, this should be enabled.
Without this things like CodeTaskFactory don't work, TaskRegistry just
throws an error.

sean-gilliam/msbuild,Microsoft/msbuild,jeffkl/msbuild,chipitsine/msbuild,Microsoft/msbuild,mono/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,mono/msbuild,cdmihai/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,Microsoft/msbuild,Microsoft/msbuild,chipitsine/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,chipitsine/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,jeffkl/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,mono/msbuild,chipitsine/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,mono/msbuild