gcc-final: Use headers in ${CLFS}/cross-tools

Adjust GCC final builds to use --with-local-prefix= and
--with-native-system-header-dir= to direct it to find headers in
${CLFS}/cross-tools/include, that's where they are now since we want to
keep header out of the final system as much as possible.
gmp,mpc,mpfr: Indicate as prereqs for GCC

It wasn't made clear before that these must be present in order to build
GCC.