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="Gustavo Zacarias <gustavo@zacarias.com.ar>"

reporter_url="http://sourceware.org/ml/crossgcc/2011-03/msg00028.html"

reporter_comment=""