[llvm] [llvm-readtapi] Add install-distribution support (PR #67748)

[Cyndy Ishida via llvm-commits]

cyndyishida wrote:

> `llvm-nm' seems special, since it is considered a binutils tool.

Yes, there are BINUTILS and CCTOOLS and TOOLCHAIN_TOOLS. I'm not opposed to adding `readtapi` to TOOLCHAIN_TOOLS but I still want to be able to install readtapi as a part of downstream `tapi`, separate from the rest of the llvm tools. 

https://github.com/llvm/llvm-project/pull/67748