gnu: make-bootstrap: Abstract things with `package-with-relocatable-glibc'.

* gnu/packages/make-bootstrap.scm (%glibc-for-bootstrap): Replace with...
  (glibc-for-bootstrap): ... this.  New procedure.
  (%standard-inputs-with-relocatable-glibc): Replace with...
  (package-with-relocatable-glibc): ... this.  New procedure.
  (%static-inputs, %gcc-static, %guile-static): Use it.