linux-user/syscall.c: Don't warn about unimplemented get_robust_list

linux-user/syscall.c: Don't warn about unimplemented get_robust_list

The nature of the kernel ABI for the get_robust_list and set_robust_list
syscalls means we cannot implement them in QEMU. Make get_robust_list
silently return ENOSYS rather than using the default "print message and
then fail ENOSYS" code path, in the same way we already do for
set_robust_list, and add a comment documenting why we do this.

This silences warnings which were being produced for emulating
even trivial programs like 'ls' in x86-64-on-x86-64.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>