build-system/gnu: Remove the "debug" output when `strip-binaries?' is #f.

* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): Delete "debug"
  from OUTPUTS when STRIP-BINARIES? is false.
* gnu/packages/cross-base.scm (cross-libc): Leave `outputs' unchanged.
  This reverts commit a4627d49.