[clang] [llvm] [RISCV] Rename TensTorrent Ascalon D8 to Ascalon X (PR #178199)

[Luke Lau via cfe-commits]

https://github.com/lukel97 commented:

Do we need backwards compatibility with -mcpu=tt-ascalon-d8? If not then it would be good to add a line in the release notes to mention that it was renamed. 

Nit in the PR title, I think Tenstorrent is spelt with the second T lowercase? 

https://github.com/llvm/llvm-project/pull/178199