std.debug: fix FP-based unwinding on powerpc64

This just needs to do the same thing as powerpc64le. Note that the saved LR is
at the same position in both ELF v1 and v2.

Author: alexrp

lib/std/debug.zig

@@ -951,7 +951,7 @@ const StackIterator = union(enum) {
 
     /// Offset of the saved return address wrt the frame pointer.
     const ra_offset = off: {
-        if (native_arch == .powerpc64le) break :off 2 * @sizeOf(usize);
+        if (native_arch.isPowerPC64()) break :off 2 * @sizeOf(usize);
         // On s390x, r14 is the link register and we need to grab it from its customary slot in the
         // register save area (ELF ABI s390x Supplement §1.2.2.2).
         if (native_arch == .s390x) break :off 14 * @sizeOf(usize);