Fix stage 2 builds with a custom libdir.
When copying libstd for the stage 2 compiler, the builder ignores the
configured libdir/libdir_relative configuration parameters. This causes
the compiler to fail to find libstd, which cause any tools built with the
stage 2 compiler to fail.
To fix this, make the copy steps of rustbuild aware of the libdir_relative
parameter when the stage >= 2. Also update the dist target to be aware of
the new location of libstd.
