Add recvmmsg and sendmmsg syscalls.

Also add the corresponding constant, struct, and function declarations
to <sys/socket.h>, and perfunctory tests so we know that the symbols
actually exist.

Signed-off-by: Guillaume Ranquet <guillaumex.ranquet@intel.com>
Change-Id: Ib0d854239d3716be90ad70973c579aff4895a4f7
diff --git a/libc/arch-x86_64/syscalls/recvmmsg.S b/libc/arch-x86_64/syscalls/recvmmsg.S
new file mode 100644
index 0000000..9da70e2
--- /dev/null
+++ b/libc/arch-x86_64/syscalls/recvmmsg.S
@@ -0,0 +1,17 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(recvmmsg)
+    movq    %rcx, %r10
+    movl    $__NR_recvmmsg, %eax
+    syscall
+    cmpq    $-MAX_ERRNO, %rax
+    jb      1f
+    negl    %eax
+    movl    %eax, %edi
+    call    __set_errno
+    orq     $-1, %rax
+1:
+    ret
+END(recvmmsg)