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/tests/Android.bp b/tests/Android.bp
index 3e1e13b..362fe39 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -68,6 +68,7 @@
         "grp_pwd_test.cpp",
         "ifaddrs_test.cpp",
         "inttypes_test.cpp",
+        "langinfo_test.cpp",
         "libc_logging_test.cpp",
         "libgen_basename_test.cpp",
         "libgen_test.cpp",