[Flang] Fix multi-config generator builds

Based on https://reviews.llvm.org/D84022 with additional changes to
maintain out-of-tree builds.

Original commit message:
Currently the binaries are output directly into the bin subdirectory of
the build directory. This doesn't work correctly with multi-config
generators which should output the binaries into <CONFIG_NAME>/bin
instead.

The original patch was implemented by David Truby and the additional
changes added here were also proposed by David Truby.

Differential Revision: https://reviews.llvm.org/D85078/



Co-authored-by: David Truby <david.truby@arm.com>