[flang] Add CMake rules to install static and shared libraries in

[flang] Add CMake rules to install static and shared libraries in
the default install directory or in the directory specified
with CMAKE_INSTALL_PREFIX.  Fixes flang-compiler/f18#359.

Original-commit: flang-compiler/f18@35ce1628e24d85eb5fa8fe467af06f7a33a4d293
Reviewed-on: https://github.com/flang-compiler/f18/pull/364