[libclc] libclc: Remove r600 support (PR #181976)

[Wenju He via cfe-commits]

https://github.com/wenju-he commented:

Following two places also need update?
https://github.com/llvm/llvm-project/blob/8f378ea7e6fa31179266c69368be56a866b631e1/libclc/clc/include/clc/math/math.h#L27
https://github.com/llvm/llvm-project/blob/8f378ea7e6fa31179266c69368be56a866b631e1/libclc/www/index.html#L40

https://github.com/llvm/llvm-project/pull/181976