[flang][runtime] Support B/O/Z editing of CHARACTER

This is a common extension, though semantics differ across
compilers.  I've chosen to interpret the CHARACTER data
as if it were an arbitrary-precision integer value and
format or read it as such.  This matches Intel's compilers
and nvfortran.  (GNU Fortran can't handle lengths > 1 and XLF
seems to get the enddianness wrong.)

This patch generalizes the previous implementations of
B/O/Z input and output so that they'll work for arbitrary data
in memory, and then uses them for all B/O/Z input/output,
including (now) CHARACTER.

Differential Revision: https://reviews.llvm.org/D124547