[flang] Roll up small fixes to runtime bugs found in testing

Summary:
Fix several bugs in the Fortran runtime found in initial
testing.

Reviewers: tskeith, PeteSteinfeld, sscalpone, jdoerfert, DavidTruby

Reviewed By: tskeith, PeteSteinfeld

Subscribers: llvm-commits, flang-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D82116