signalfd.h: Use new "bits/" scheme for arch-specific flags

signalfd.h: Use new "bits/" scheme for arch-specific flags

glibc has split off the arch-specific flag definitions (like
SFD_NONBLOCK) into <arch>/bits/signalfd.h, so that there are no longer
multiple redundant copies of the entire header file.  We will adopt the
same scheme in uClibc.

Special cases are included for: alpha mips sparc

hppa was omitted because it has not been updated in glibc.

All others use the common definition.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>