Correctly pass on LLVM_DEFINITIONS to CMake

What reproduces the bug? Provide code if possible.

bpftrace fails to build when LLVM_DEFINITIONS contains multiple definitions separated by spaces.

https://github.com/llvm/llvm-project/commit/a2a65a5bae3c503d40c219ef9ba460b1fb34d1bf explains how to deal with this properly:

  separate_arguments(LLVM_DEFINITIONS_LIST NATIVE_COMMAND ${LLVM_DEFINITIONS})
  add_definitions(${LLVM_DEFINITIONS_LIST})