Elide use of separate LLVM branch for musl builds

[compor]

Maintaining a separate branch for musl builds is a bit cumbersome, especially now that there is support for conditional feature inclusion of TableGen configurations.

I looked at the commits in the musl branch of llvm and:

This branch sets up:

1. X86 calling conventions for callee saved registers.

2. AArch64 RegisterInfo for the general purpose registers of 32 and 64 bits, excluding the FP regs

3. AArch64 calling conventions

4. This is under the UNIFICO_GPR_CALLING_CONV TableGen preprocessing flag on the development branch.

5. This is also under UNIFICO_GPR_CALLING_CONV TableGen preprocessing as 1.

6. Same as 1 and 2 above.

Hence, the musl branch is just the TableGen changes with UNIFICO_GPR_CALLING_CONV defined and everything else Unifico-specific disabled.

@blackgeorge-boom Can you please verify the above?

If so, I'll adapt the build scripts, documentation on them and the CMake options and maybe go on with a Make preset for the rest of the flags.

[blackgeorge-boom]

In theory, this should work.

I think we can go ahead with this and see if it works in practice (I am only unsure about the commit blackgeorge-boom/llvm-project@fc96e4c that we don't have in the development branch, but have the whole popcorn patch instead).