linux: Fix __NSIG_WORDS and add __NSIG_BYTES

The __NSIG_WORDS value is based on minimum number of words to hold
the maximum number of signals supported by the architecture.

This patch also adds __NSIG_BYTES, which is the number of bytes
required to represent the supported number of signals.  It is used in
syscalls which takes a sigset_t.

Checked on x86_64-linux-gnu and i686-linux-gnu.

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