build-system/gnu: Strip with '--strip-all' instead of '--strip-debug'.

This saves 19% on the 'bin' directory of Coreutils, and certainly
helpful for things like Git's 'libexec' directory.

* guix/build-system/gnu.scm (gnu-build): Change default value for
  #:strip-flags to '("--strip-all").
* guix/build/gnu-build-system.scm (strip): Ditto.
* gnu/packages/linux.scm (linux-libre)[arguments]: Add #:strip-flags.