powerpc/signal: Fix possible build failure with unsafe_copy_fpr_{to/from}_user

When neither CONFIG_VSX nor CONFIG_PPC_FPU_REGS are selected,
unsafe_copy_fpr_to_user() and unsafe_copy_fpr_from_user() are
doing nothing.

Then, unless the 'label' operand is used elsewhere, GCC complains
about it being defined but not used.

To fix that, add an impossible 'goto label'.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/cadc0a328bc8e6c5bf133193e7547d5c10ae7895.1620465920.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/signal.h

@@ -166,9 +166,9 @@ copy_ckfpr_from_user(struct task_struct *task, void __user *from)
 }
 #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
 #else
-#define unsafe_copy_fpr_to_user(to, task, label) do { } while (0)
+#define unsafe_copy_fpr_to_user(to, task, label) do { if (0) goto label;} while (0)
 
-#define unsafe_copy_fpr_from_user(task, from, label) do { } while (0)
+#define unsafe_copy_fpr_from_user(task, from, label) do { if (0) goto label;} while (0)
 
 static inline unsigned long
 copy_fpr_to_user(void __user *to, struct task_struct *task)