[llvm] [clang] [PowerPC] Implement fence builtin (PR #76495)

[Chen Zheng via llvm-commits]

================
@@ -944,6 +944,9 @@ TARGET_BUILTIN(__builtin_pack_vector_int128, "V1LLLiULLiULLi", "", "vsx")
 // Set the floating point rounding mode
 BUILTIN(__builtin_setrnd, "di", "")
 
+// Barrier for instruction motion
----------------
chenzheng1030 wrote:

Can we add some comments here to emphasize that `__fence` will stop code motion for register based instructions?

https://github.com/llvm/llvm-project/pull/76495