[lld] [ELF][AArch64] Do not treat missing build attributes as defined (PR #213600)

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

Thanks!

> I think there's a good case for making it add the build attribute if it isn't already there.

This might be a bit tricky because it is possible to define conflicting custom properties with `.aeabi_subsection` and `.aeabi_attribute` instructions. But this discussion is for another PR.

https://github.com/llvm/llvm-project/pull/213600