fix generating syscall-names.h

fix generating syscall-names.h

Sometimes the syscall number is not defined with a number but with an
offset to an other syscall and then make_syscall_h.sh created some
broken header file.

For example the bit/syscall.h from musl for i386 has this:

  #define __NR_timer_create     259
  #define __NR_timer_settime    (__NR_timer_create+1)

With this patch the resulting array looks like this:

 [259] = "timer_create",
 [(__NR_timer_create+1)] = "timer_settime",

This fixes this bug from OpenWrt:
https://dev.openwrt.org/ticket/20195

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>