sh: bring syscall_set_return_value in line with other architectures

Other architectures expect that syscall_set_return_value gets an already
negative value as error. That's also what kernel/seccomp.c provides.

Signed-off-by: Michael Karcher <kernel@mkarcher.dialup.fu-berlin.de>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Rich Felker <dalias@libc.org>