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

[Andrew Haberlandt via llvm-commits]

ndrewh wrote:

@smeenai This is a good point. If we are set on the current (silly) behavior of both appending and prepending these options, then we will have to keep as-is.

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