libc/glibc: do not overwrite existing bits/syscall.h

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Rafael C <groups.r2@gmail.com>
Cc: Jérôme BARDON <bardon.pro@gmail.com>
Cc: Daniel Price <daniel.price@gmail.com>

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="s390x${CT_ARCH_SUFFIX}"

     fi

 }