clang/LLVM is a bit more finicky with what target triples it accepts compared
to GCC. However, the GCC target triple is needed to properly locate the C
libraries. In case a locally installed GCC toolchain uses a technically not
fully correct target triple, it might be impossible to build with LLVM.
The solution here is to allow specifying a TARGET_ARCH_LLVM target triple
(which defaults to TARGET_ARCH), which is passed to clang. For locating the
C lib, still TARGET_ARCH is used.
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