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/Android.mk b/libc/Android.mk
index edf834b..45297b5 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -284,6 +284,7 @@
     bionic/raise.cpp \
     bionic/__set_errno.cpp \
     bionic/setlocale.cpp \
+    bionic/signalfd.cpp \
     bionic/__strcat_chk.cpp \
     bionic/__strcpy_chk.cpp \
     bionic/strerror.cpp \