powerpc: Only pad struct pt_regs when needed

If neither KUAP nor PPC64 is selected, there is nothing in the second
union of struct pt_regs, so the alignment padding is waste of memory.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d536bbc46094f66b24d3017343be25164f232933.1620307840.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/ptrace.h

@@ -47,7 +47,7 @@ struct pt_regs
 			unsigned long result;
 		};
 	};
-
+#if defined(CONFIG_PPC64) || defined(CONFIG_PPC_KUAP)
 	union {
 		struct {
 #ifdef CONFIG_PPC64
@@ -67,6 +67,7 @@ struct pt_regs
 		};
 		unsigned long __pad[4];	/* Maintain 16 byte interrupt stack alignment */
 	};
+#endif
 };
 #endif