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 0c11611c11, modified to
work with older versions of GCC still in use on some branches.)

Change-Id: I4c6c3f12199559af8be63f93a5336851b7e63355

libc/arch-arm/syscalls/signalfd4.S

@@ -0,0 +1,14 @@
+/* autogenerated by gensyscalls.py */
+#include <machine/asm.h>
+#include <sys/linux-syscalls.h>
+
+ENTRY(signalfd4)
+    .save   {r4, r7}
+    stmfd   sp!, {r4, r7}
+    ldr     r7, =__NR_signalfd4
+    swi     #0
+    ldmfd   sp!, {r4, r7}
+    movs    r0, r0
+    bxpl    lr
+    b       __set_syscall_errno
+END(signalfd4)