[llvm] [runtimes][CMake] Move Fortran support code from flang-rt (PR #171610)

[Petr Hosek via llvm-commits]

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petrhosek wrote:

The compiler-rt version is used by the builtins build which runs separately from the rest of the runtimes. If/when we fold builtins into the runtimes, we could remove the compiler-rt copy.

https://github.com/llvm/llvm-project/pull/171610