[llvm] Add support for CHERI "purecap" as an environment in target triples. (PR #118927)

[Alexander Richardson via llvm-commits]

arichardson wrote:

While I i initially added this for MIPS I'm not 100% convinced it's the best approach to use an environment here as we might want the existing ones too. I'm starting to think using an architecture like riscv64c might be cleaner an less error prone. @jrtc27 what do you think?

https://github.com/llvm/llvm-project/pull/118927