[llvm] [runtimes] Fix duplicate BUILTINS_CMAKE_ARGS/RUNTIMES_CMAKE_ARGS (PR #160631)

[Dan Blackwell via llvm-commits]

DanBlackwell wrote:

I believe that `-D` can take precedence over a cachefile if the definition inside is not `FORCE`d (https://cmake.org/cmake/help/latest/manual/cmake.1.html#cmdoption-cmake-D). That being said, I feel as though removing the duplication by eliminating the 'prepend' would be safer WRT existing workflows - is that possible?

https://github.com/llvm/llvm-project/pull/160631