ARM: entry: allow ARM-private syscalls to be restarted

System calls will only be restarted after signal handling if they (a)
return an error code indicating that a restart is required and (b) have
`why' set to a non-zero value, to indicate that the signal interrupted
them.

This patch leaves `why' set to a non-zero value for ARM-private syscalls
, and only zeroes it for syscalls that are not implemented.

Signed-off-by: Will Deacon <will.deacon@arm.com>