gnu: coreutils: Fix cross-compilation.

gnu: coreutils: Fix cross-compilation.

Previously cross-compiling coreutils would fail with:

  aarch64-linux-gnu-gcc   -Os -g0 -Wl,--as-needed -static -pthread -o src/uptime src/uptime.o src/libver.a lib/libcoreutils.a  lib/libcoreutils.a
  /gnu/store/hk8615wmmzk28k5z06g69jm6vbhc8lgk-glibc-cross-aarch64-linux-gnu-2.26.105-g0890d5379c-static/lib/libc.a(mktime.o): In function `__mktime_internal':
  /tmp/guix-build-glibc-cross-aarch64-linux-gnu-2.26.105-g0890d5379c.drv-0/glibc-2.26-105-g0890d5379c/time/mktime.c:353: multiple definition of `__mktime_internal'
  lib/libcoreutils.a(mktime.o):mktime.c:(.text+0x214): first defined here
  collect2: error: ld returned 1 exit status

* gnu/packages/base.scm (coreutils)[arguments]: Add #:configure-flags
when cross-compiling.