op.h: Add additional padding to struct opslab to ensure proper alignment

On m68k, the natural alignment is 16 bits which causes the opslab_opslot
member of struct opslab to be aligned at a 16-bit offset. Other 32-bit
and 64-bit architectures have a natural alignment of at least 32 bits, so
the offset is always guaranteed to be at least 32-bit-aligned.

Fix this by adding additional padding bytes before the opslab_opslot
member, both for cases when PERL_DEBUG_READONLY_OPS defined and not
defined to ensure the offset of oplab_slots is always 32-bit-aligned.
On architectures which have a natural alignment of at least 32 bits,
the padding does not affect the alignment, offsets or struct size.

op.h

@@ -713,6 +713,9 @@ struct opslab {
                                            units) */
 # ifdef PERL_DEBUG_READONLY_OPS
     bool	opslab_readonly;
+    U8          opslab_padding;         /* padding to ensure that opslab_slots is always */
+# else
+    U16         opslab_padding;         /* located at an offset with 32-bit alignment */
 # endif
     OPSLOT	opslab_slots;		/* slots begin here */
 };