[flang] Make sure that `flang` is also installed

This patch makes sure that `flang` (symlink to `flang-to-external-fc`)
is installed alongside other Flang tools. Fixes build failures in
clang-aarch64-full-2stage [1] introduced after merging
https://reviews.llvm.org/D125832.

[1] https://lab.llvm.org/buildbot/#/builders/179/builds/3799

Differential Revision: https://reviews.llvm.org/D126760