Add signalfd call to bionic

Add signalfd() call to bionic.

Adding the signalfd call was done in 3 steps:
- add signalfd4 system call (function name and syscall
  number) to libc/SYSCALLS.TXT
- generate all necessary headers by calling
  libc/tools/gensyscalls.py. This patch is adding
  the generated files since the build system
  does not call gensyscalls.py.
- create the signalfd wrapper in signalfd.cpp and add
  the function prototype to sys/signalfd.h

(cherry-pick of 0c11611c11f4dc1b6d43587b72c3ccbe8c51a51c, modified to
work with older versions of GCC still in use on some branches.)

Change-Id: I4c6c3f12199559af8be63f93a5336851b7e63355
Add SOCK_CLOEXEC and SOCK_NONBLOCK socket flags.

SOCK_CLOEXEC is used to atomically set close-on-exec flag for the new
descriptor(s), and SOCK_NONBLOCK is used to mark descriptor(s) as
non-blocking.

Change-Id: I8ba6a70543d23759e3ddcc7ff9c21b567184d681