utils: Add `mkdir-p'; use it.

* guix/build/utils.scm (mkdir-p): New procedure.

* distro/packages/base.scm (gnu-make-boot0, gcc-boot0-wrapped,
  ld-wrapper-boot3, %static-binaries, %guile-static-stripped): Use it.
* distro/packages/typesetting.scm (lout): Likewise.