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-arm64/syscalls/sendmmsg.S b/libc/arch-arm64/syscalls/sendmmsg.S
new file mode 100644
index 0000000..43b2fbf
--- /dev/null
+++ b/libc/arch-arm64/syscalls/sendmmsg.S
@@ -0,0 +1,21 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(sendmmsg)
+    stp     x29, x30, [sp, #-16]!
+    mov     x29,  sp
+    str     x8,       [sp, #-16]!
+
+    mov     x8, __NR_sendmmsg
+    svc     #0
+
+    ldr     x8,       [sp], #16
+    ldp     x29, x30, [sp], #16
+
+    cmn     x0, #(MAX_ERRNO + 1)
+    cneg    x0, x0, hi
+    b.hi    __set_errno
+
+    ret
+END(sendmmsg)