Implementing a new LLVM based toolchain

[Crivella]

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Compilers

• New llvm.py compiler taken from the original clang.py and unifying it with flang
  • Now allows defining a list of unsupported flang compiler options based on the passed LLVM version which will be excluded, from the compiler flags

Toolchains

• LLVMtc: Similar to GCC uses bot GCCcore and SYSTEM as a subtoolchain
• Lfbf equivalent of Gfbf
• Lompi equivalent of Gompi
• LFoss equivalent of Foss
  • Does not have the OpenBLAS child class as in Foss it is used only for toolchains older than 2021.0

TODO

• [ ] Extensively investigate+test the comments related to the compiler flags on the old clang.py compiler
• [ ] Extensively test interplay of compilations with libstdc++ and possibly libc++
• [ ] Interplay between LLVM and CUDA / ROCm
• [ ] Possibly have variants of LLVM like ROCm-LLVM work through the same toolchain