Fix Rust lex selftest when using libiconv

The Rust lex selftest fails on our Windows build.  I tracked this down
to a use of UTF-32 as a parameter to convert_between_encodings.  Here,
iconv_open succeeds, but the actual conversion of a tab character
fails with EILSEQ.  I suspect that "UTF-32" is being interpreted as
big-endian, as changing the call to use "UTF-32LE" makes it work.
This patch implements this fix.