[flang][runtime] Fix padding in CHARACTER(4) assignments.

One piece of pointer arithmetic was adding the number of bytes instead
of the number of characters. This caused failures in CHARACTER(KIND>1)
that required padding.
This was caught using HLFIR that currently uses the runtime for array
assignment where the current lowering does everything inline.

Reviewed By: vzakhari, klausler

Differential Revision: https://reviews.llvm.org/D149062