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-arm/syscalls/sendmmsg.S b/libc/arch-arm/syscalls/sendmmsg.S
new file mode 100644
index 0000000..04c3fe3
--- /dev/null
+++ b/libc/arch-arm/syscalls/sendmmsg.S
@@ -0,0 +1,14 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(sendmmsg)
+    mov     ip, r7
+    ldr     r7, =__NR_sendmmsg
+    swi     #0
+    mov     r7, ip
+    cmn     r0, #(MAX_ERRNO + 1)
+    bxls    lr
+    neg     r0, r0
+    b       __set_errno
+END(sendmmsg)