RISC-V: Likely mixup with headers and libraries used from two different LLVM in the system

[pjaaskel]

It manifests as linkage time errors like this when building the newlib's default RISC-V plugin. A snippet from the huge error log:

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/usr/bin/ld: RISCVISelLowering.cpp:(.text+0x21e8a): undefined reference to `llvm::MetadataTracking::retrack(void*, llvm::Metadata&, void*)'
/usr/bin/ld: RISCVISelLowering.cpp:(.text+0x21f2a): undefined reference to `llvm::MachineInstr::addOperand(llvm::MachineFunction&, llvm::MachineOperand const&)'
/usr/bin/ld: RISCVISelLowering.cpp:(.text+0x21f6a): undefined reference to `llvm::MachineInstr::addOperand(llvm::MachineFunction&, llvm::MachineOperand const&)'
/usr/bin/ld: RISCVISelLowering.cpp:(.text+0x21f81): undefined reference to `llvm::MetadataTracking::untrack(void*, llvm::Metadata&)'
/usr/bin/ld: RISCVISelLowering.cpp:(.text+0x21f98): undefined reference to `llvm::MetadataTracking::untrack(void*, llvm::Metadata&)'
/usr/bin/ld: RISCVISelLowering.cpp:(.text+0x21fc3): undefined reference to `llvm::MachineRegisterInfo::createVirtualRegister(llvm::TargetRegisterClass const*, llvm::StringRef)'
/usr/bin/ld: RISCVISelLowering.cpp:(.text+0x22006): undefined reference to `llvm::MetadataTracking::track(void*, llvm::Metadata&, llvm::PointerUnion<llvm::MetadataAsValue*, llvm::Metadata*>)'