Skip f16 to f128 conversion test for ppc64

As for aarch64 we're waiting for LLVM to emit calls to the specific
builtins that implement this conversion.

test/stage1/behavior/widening.zig

@@ -31,6 +31,7 @@ test "float widening" {
 test "float widening f16 to f128" {
     // TODO https://github.com/ziglang/zig/issues/3282
     if (@import("builtin").arch == .aarch64) return error.SkipZigTest;
+    if (@import("builtin").arch == .powerpc64le) return error.SkipZigTest;
 
     var x: f16 = 12.34;
     var y: f128 = x;