stdenv: force gmp to rebuild in stage4 of the bootstrap
stdenv: force gmp to rebuild in stage4 of the bootstrap

As explained in the comment, this ensures that stage4-coreutils does
not leak a reference to the bootstrap-files by way of libgmp.  This
will allow the next patch in this series to build stage4-coreutils
using a dynamically-linked (rather than statically-linked) libgmp.

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