Fix final binaries leaking build paths

[kilyanni]

A small irreproducibility issue I noticed while packaging this with Nix:

The final binaries leak paths to the build directory, meaning binaries aren't directly bit-for-bit reproducible.
This PR adds path maps so that the build is idempotent over the build directory path & clang resource dir path.

Should not change behavior.

[Copilot]

Pull request overview

This PR addresses build irreproducibility where absolute build-directory and LLVM install paths can leak into produced archives (via debug-info paths), preventing bit-for-bit reproducible outputs across different build environments. It does so by adding deterministic path remapping flags to the CMake builds for compiler-rt and libc++.

Changes:

• Detects Clang’s resource directory at build time and defines -ffile-prefix-map remappings for the repo root and resource dir.
• Applies those remapping flags to CMake C/C++/ASM flags for the compiler-rt build.
• Applies those remapping flags to CMake C/C++/ASM flags for the libcxx (LLVM runtimes) build.

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