libjulia_jll: turn LibOSXUnwind_jll into a build dep for Julia 1.6

[fingolfin]

This is experimental. I still need to verify that the result can
be used to build other JLLs using libjulia_jll. But I thought
it'd be good to already collect feedback here.

[skip ci] [skip build]

[fingolfin]

Thanks for working on this @giordano. I've renamed my patch to hopefully fix the cd: too many arguments error. That leaves KeyError: key UUID("a83860b7-747b-57cf-bf1f-3e79990d037f") not found for Julia 1.6.

[giordano]

Yeah, this is becoming a circular issue...

[fingolfin]

:-(.

next idea then: manually download and extract the libunwindOSX artifact, remove it as a builddep. But need sleep first

[fingolfin]

@giordano how can I fix this again?

ERROR: LoadError: Unsatisfiable requirements detected for package OpenBLAS_jll [4536629a]:
 OpenBLAS_jll [4536629a] log:
 ├─possible versions are: 0.3.13 or uninstalled
 └─restricted to versions 0.3.10 by an explicit requirement — no versions left

[giordano]

BinaryBuilderBase.get_addable_spec, I presume.

[fingolfin]

sigh

ERROR: LoadError: Unsatisfiable requirements detected for package libLLVM_jll [8f36deef]:
 libLLVM_jll [8f36deef] log:
 ├─possible versions are: 12.0.1 or uninstalled
 └─restricted to versions 11.0.1 by an explicit requirement — no versions left

[giordano]

Yeah, they all need to become get_addable_spec. Every time we move forward with the Julia used for the build, old dependences may become uninstallable otherwise.

[fingolfin]

This should be ready. But perhaps should be merged with skip ci, skip build, and then we focus on PR #4461.

[giordano]

Thanks a lot Max!