build-system/gnu: Unify with (guix build-system gnu-cross-build).
* guix/build/gnu-build-system.scm (set-paths): Add `native-inputs' and
`native-search-paths' keyword parameters. Honor them.
(configure): Add `target' and `native-inputs' keyword parameters.
Look for Bash in NATIVE-INPUTS or INPUTS. Pass `--host' when TARGET
is true.
(strip): Add `strip-command' keyword parameter. Use it.
* guix/build/gnu-cross-build.scm: Remove.
* Makefile.am (MODULES): Adjust accordingly.
* gnu/packages/acl.scm, gnu/packages/attr.scm, gnu/packages/base.scm,
gnu/packages/bash.scm, gnu/packages/gawk.scm,
gnu/packages/gettext.scm, gnu/packages/guile.scm,
gnu/packages/libffi.scm, gnu/packages/libsigsegv.scm,
gnu/packages/linux.scm, gnu/packages/ncurses.scm,
gnu/packages/readline.scm, guix/build-system/gnu.scm: Replace
`%standard-cross-phases' by `%standard-phases'. Remove references
to (guix build gnu-cross-build).