Add a standalone derivation for LLVM's flang-rt runtime libraries
(libFortranRuntime, libFortranDecimal). flang-rt is a runtimes-style
package: building it requires running a working flang binary at *build*
time, but the produced libraries are linked into derivations targeting
the *host*.
The runtime is therefore wired up using `buildFlang`, sourced from
`buildLlvmPackages.flang-unwrapped` so that the build-platform flang
(spliced via the LLVM package set) compiles the runtime, while the
resulting libraries are still produced for the host platform. This
mirrors the existing `buildLlvmTools.tblgen` / `buildLlvmPackages.tblgen`
pattern used elsewhere in the LLVM package set.
`unwrapped` rather than the wrapped flang is used because flang-rt's
CMake glue invokes the compiler directly and does not need (or want)
Nix's cc-wrapper resource-dir and rpath plumbing during the runtime
build.
flang-rt only consumes LLVM's CMake helpers and the build-time flang;
it does not link against MLIR or libclang, so neither MLIR_DIR nor
CLANG_DIR is passed and the unused empty `dev` output is omitted.
Co-authored-by: stove <stove@rivosinc.com>
Co-authored-by: acture <acture@gmail.com>
Acture
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