signal: Add signum-{generic,arch}.h

It refactor how signals are defined by each architecture.  Instead of
include a generic header (bits/signum-generic.h) and undef non-default
values in an arch specific header (bits/signum.h) the new scheme uses a
common definition (bits/signum-generic.h) and each architectures add
its specific definitions on a new header (bits/signum-arch.h).

For Linux it requires copy some system default definitions to alpha,
hppa, and sparc.  They are historical values and newer ports uses
the generic Linux signum-arch.h.

For Hurd the BSD signum is removed and moved to a new header (it is
used currently only on Hurd).

Checked on a build against all affected ABIs.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>