Implement <iconv.h>.

Bug: http://b/32978596
Test: ran tests
Change-Id: I56b6ae3d9c5a3a56d2b4afba33fb8f9e964bf7b9
diff --git a/libc/libc.x86.map b/libc/libc.x86.map
index 34370ba..567a5bf 100644
--- a/libc/libc.x86.map
+++ b/libc/libc.x86.map
@@ -1305,6 +1305,9 @@
     __freading; # future
     __fwriting; # future
     getlogin_r; # future
+    iconv; # future
+    iconv_close; # future
+    iconv_open; # future
     syncfs; # future
 } LIBC_O;