Fixed an error in xbuild_targets.make when using --with-profile2=no or --with-profile4=no

When turning off the net_2_0 or net_4_0 profile the respective folder in mcs/class/lib/ doesn't exist,
which results in an error when trying to copy a file to it in xbuild_targets.make

This occurs for example when running unit tests in ```mcs/class/Microsoft.Build.Framework```