[clang] [llvm] [PowerPC][X86] Make cpu id builtins target independent and lower for PPC (PR #68919)

[Kamau Bridgeman via cfe-commits]

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@@ -210,6 +210,12 @@ let TargetPrefix = "ppc" in {  // All intrinsics start with "llvm.ppc.".
             [llvm_float_ty],
             [llvm_float_ty, llvm_float_ty, llvm_float_ty, llvm_vararg_ty],
             [IntrNoMem]>;
+  // Load of a value provided by the system library at a fixed address. Used for
+  // accessing things like HWCAP word provided by GLIBC.
----------------
kamaub wrote:

```suggestion
// This intrinsic is provided to allow back ends to emit load
// instructions that load a value from a fixed address. The
// parameter to the intrinsic is not an address, but an
// immediate index into an enumeration that contains the
// union of all such values available on all back ends.
// An example is the HWCAP/HWCAP2/CPUID words
// provided by GLIBC on PowerPC to allow fast access
// to commonly used parts of AUXV. These are provided
// at a fixed offset into the TCB (accessible through the
// thread pointer).
```
Missing expanded comment from previous round of reviews in https://reviews.llvm.org/D152914

https://github.com/llvm/llvm-project/pull/68919