Implement <langinfo.h>.

POSIX locale only, as usual.

The GNU YESSTR and NOSTR extensions return the empty string in the C locale,
so I haven't bothered supporting them.

Bug: http://b/1401872
Test: bionic tests
Change-Id: I6846839e4f9f1812344ed5dce0b93f83c0c20eb3
diff --git a/libc/libc.arm64.map b/libc/libc.arm64.map
index 28755d4..179d8bf 100644
--- a/libc/libc.arm64.map
+++ b/libc/libc.arm64.map
@@ -1210,6 +1210,8 @@
     msgget; # future
     msgrcv; # future
     msgsnd; # future
+    nl_langinfo; # future
+    nl_langinfo_l; # future
     pthread_getname_np; # future
     quotactl; # future
     semctl; # future