Add preadv/pwritev.

Bug: http://b/12612572
Change-Id: I38ff2684d69bd0fe3f21b1d371b88fa60d5421cb
diff --git a/libc/arch-x86_64/syscalls/pwritev.S b/libc/arch-x86_64/syscalls/pwritev.S
new file mode 100644
index 0000000..d3bd8fa
--- /dev/null
+++ b/libc/arch-x86_64/syscalls/pwritev.S
@@ -0,0 +1,18 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(pwritev)
+    movq    %rcx, %r10
+    movl    $__NR_pwritev, %eax
+    syscall
+    cmpq    $-MAX_ERRNO, %rax
+    jb      1f
+    negl    %eax
+    movl    %eax, %edi
+    call    __set_errno_internal
+1:
+    ret
+END(pwritev)
+
+ALIAS_SYMBOL(pwritev64, pwritev)