cc/gcc: add 'cflags' paramater to the core backend

As the core backend is used to generate the bare-metal compiler,
we need to pass it the host CFLAGS.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 #!/bin/sh

 set -e

 printf "Running autoconf...\n"

 autoconf -Wall --force

 printf "Done. You may now run:\n    ./configure\n"