RISC-V: Cleanup some of the sysdep.h code

Remove a duplicate inclusion of <sysdeps/unix/sysdep.h> which is already
pulled via <sysdeps/unix/sysv/linux/generic/sysdep.h>, and the inclusion
of <errno.h> whose definition of `__set_errno' is not needed here.

Reviewed-by: Maciej W. Rozycki <macro@wdc.com>