gnu: make-bootstrap: Allow cross-compilation of the bootstrap tarballs.

gnu: make-bootstrap: Allow cross-compilation of the bootstrap tarballs.

* gnu/packages/make-bootstrap.scm (package-with-relocatable-glibc): Turn
  `inputs' into a thunk.  Add `native-inputs', and pass it to
  `package-with-explicit-inputs'.
  (%static-inputs): Add %BASH-STATIC as an input to gawk when cross
  building.
  (%glibc-stripped): When cross compiling,
  use (glibc-for-bootstrap (cross-libc)) as the basis.
  (%gcc-static): Base on GCC-4.7, not GCC-FINAL.
  (tarball-package): TAR and XZ are native inputs.  Choose the tarball
  file name based on (%current-target-system) too.