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-mips/syscalls/recvmmsg.S b/libc/arch-mips/syscalls/recvmmsg.S
new file mode 100644
index 0000000..23e511b
--- /dev/null
+++ b/libc/arch-mips/syscalls/recvmmsg.S
@@ -0,0 +1,19 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(recvmmsg)
+    .set noreorder
+    .cpload t9
+    li v0, __NR_recvmmsg
+    syscall
+    bnez a3, 1f
+    move a0, v0
+    j ra
+    nop
+1:
+    la t9,__set_errno
+    j t9
+    nop
+    .set reorder
+END(recvmmsg)