[flang] Fix multi-config generator builds.

Summary:
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.

Reviewers: sscalpone, richard.barton.arm

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84022