Switch sigpending over to rt_sigpending.

Change-Id: I7b28984796b5fb343cfbcc47e0afc3a84293d417

libc/arch-x86_64/syscalls/__rt_sigpending.S

@@ -0,0 +1,17 @@
+/* autogenerated by gensyscalls.py */
+#include <asm/unistd.h>
+#include <linux/err.h>
+#include <machine/asm.h>
+
+ENTRY(__rt_sigpending)
+    movl    $__NR_rt_sigpending, %eax
+    syscall
+    cmpq    $-MAX_ERRNO, %rax
+    jb      1f
+    negl    %eax
+    movl    %eax, %edi
+    call    __set_errno
+    orq     $-1, %rax
+1:
+    ret
+END(__rt_sigpending)