[flang] Always encode multi-byte output in UTF-8

A recent change to implement UTF-8 encoding should have
made the encoding conditional only for CHARACTER(KIND=1)
to enable UTF-8 output vs. Latin-1 or whatever.  UTF-8 output
of wider CHARACTER kinds should not be conditional (until we choose
to support UCS-16, maybe).  So wider CHARACTER kinds are being
emitted with extra zero bytes; this patch fixes them.

Differential Revision: https://reviews.llvm.org/D123711