signal: Document glibc's si_code of SI_ASYNCNL

The header uapi/asm-generic/siginfo.h appears to the the repository of
all of these definitions in linux so collect up glibcs additions as
well.  Just to prevent someone from accidentally creating a conflict
in the future.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>