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/arch-x86/syscalls/signalfd4.S b/libc/arch-x86/syscalls/signalfd4.S
new file mode 100644
index 0000000..f1c2ea0
--- /dev/null
+++ b/libc/arch-x86/syscalls/signalfd4.S
@@ -0,0 +1,32 @@
+/* autogenerated by gensyscalls.py */
+#include <sys/linux-syscalls.h>
+
+    .text
+    .type signalfd4, @function
+    .globl signalfd4
+    .align 4
+
+signalfd4:
+    pushl   %ebx
+    pushl   %ecx
+    pushl   %edx
+    pushl   %esi
+    mov     20(%esp), %ebx
+    mov     24(%esp), %ecx
+    mov     28(%esp), %edx
+    mov     32(%esp), %esi
+    movl    $__NR_signalfd4, %eax
+    int     $0x80
+    cmpl    $-129, %eax
+    jb      1f
+    negl    %eax
+    pushl   %eax
+    call    __set_errno
+    addl    $4, %esp
+    orl     $-1, %eax
+1:
+    popl    %esi
+    popl    %edx
+    popl    %ecx
+    popl    %ebx
+    ret