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
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 2973caa..dc139bd 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -225,6 +225,7 @@
 int     __rt_sigprocmask:rt_sigprocmask (int  how, const sigset_t *set, sigset_t *oset, size_t sigsetsize)  175,175,195
 int     __rt_sigtimedwait:rt_sigtimedwait(const sigset_t *set, struct siginfo_t  *info, struct timespec_t  *timeout, size_t  sigset_size)  177,177,197
 int     sigpending(sigset_t *)  73
+int     signalfd4(int fd, const sigset_t *mask, size_t sizemask, int flags)  355,327,324
 
 # sockets
 int           socket(int, int, int)              281,-1,183