Enable the AVR backend of LLVM

[Seelengrab]

This PR enables the AVR backend of LLVM. @vchuravy mentioned on Slack I should open this, to make it easier for others debugging AVRCompiler/GPUCompiler related issues so they don't have to build LLVM locally. The backend itself is not that large (1.5MB), a bit smaller than BPF (1.8MB):

[sukera@tower lib]$ du -shc libLLVMBPF*
60K	libLLVMBPFAsmParser.a
1.6M	libLLVMBPFCodeGen.a
156K	libLLVMBPFDesc.a
20K	libLLVMBPFDisassembler.a
12K	libLLVMBPFInfo.a
1.8M	total
[sukera@tower lib]$ du -shc libLLVMAVR*
76K	libLLVMAVRAsmParser.a
1.1M	libLLVMAVRCodeGen.a
316K	libLLVMAVRDesc.a
20K	libLLVMAVRDisassembler.a
8.0K	libLLVMAVRInfo.a
1.5M	total

This does NOT mean that julia itself builds with AVR or is supported to run in any support tier other than "there is no support".

[gbaraldi]

This won't change the version from BinaryBuilder but I'm going to add it when I next rebuild it.

[giordano]

@vchuravy I guess we may want to add it to the spack recipe as well: https://github.com/spack/spack/blob/85d51bfd9a3168f181388421c1d22c8678ecaff9/var/spack/repos/builtin/packages/julia/package.py#L52?

[vchuravy]

Yeah eventually for Julia 1.10

[vchuravy]

@KristofferC this change accidentally snuck into LLVM 14.0.6+3 JuliaPackaging/Yggdrasil#6615

I could either rebuild LLVM again with AVR disabled or we backport this PR.