[flang] Make runtimes headers independent of the main llvm headers

llvm/Support/Compiler.h requires llvm-config.h, i.e. to include it,
you'd need to actually set up building llvm for that target.

If using flang to cross compile code for a different target, we need
the runtimes built for that target, but we might not want to build
llvm itself for that target.

This is 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/D130351