[flang] Allow configuring building the flang runtime standalone

This allows building the flang runtime and libFortranDecimal standalone,
without setting up the full llvm build, by pointing cmake to these
directories.

This is the other one out of two essential steps for building the flang runtimes
for a foreign target, i.e. setting up cross compiling with flang.

Differential Revision: https://reviews.llvm.org/D130352