[CMake] Use LLVM_TARGET_TRIPLE in runtimes

This variable is derived from LLVM_DEFAULT_TARGET_TRIPLE by default,
but using a separate variable allows additional normalization to be
performed if needed.

Differential Revision: https://reviews.llvm.org/D137451

GitOrigin-RevId: bec8a372fc0db95852748691c0f4933044026b25
1 file changed