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>

 reporter_name="Remy Bohmer"

 reporter_url="linux@bohmer.net"

 reporter_comment="Example for building a toolchain for m68000 bare metal targets"