powerpc: Reverse stack frame marker on little endian

On little endian the stack frame marker appears reversed when dumping
memory sequentially, as is typical in xmon or gdb, eg:

  c000000004733e40 0000000000000000 0000000000000000  |................|
  c000000004733e50 0000000000000000 0000000000000000  |................|
  c000000004733e60 0000000000000000 0000000000000000  |................|
  c000000004733e70 5347455200000000 0000000000000000  |SGER............|
  c000000004733e80 a700000000000000 708897f7ff7f0000  |........p.......|
  c000000004733e90 0073428fff7f0000 208997f7ff7f0000  |.sB..... .......|
  c000000004733ea0 0100000000000000 ffffffffffffffff  |................|
  c000000004733eb0 0000000000000000 0000000000000000  |................|

To make it easier to recognise, reverse the value on little endian, so
it always appears as "REGS", eg:

  c000000004733e70 5245475300000000 0000000000000000  |REGS............|
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220927150419.1503001-2-mpe@ellerman.id.au

arch/powerpc/include/asm/ptrace.h

@@ -99,7 +99,12 @@ struct pt_regs
 
 #define STACK_FRAME_WITH_PT_REGS (STACK_FRAME_OVERHEAD + sizeof(struct pt_regs))
 
+// Always displays as "REGS" in memory dumps
+#ifdef CONFIG_CPU_BIG_ENDIAN
 #define STACK_FRAME_REGS_MARKER	ASM_CONST(0x52454753)
+#else
+#define STACK_FRAME_REGS_MARKER	ASM_CONST(0x53474552)
+#endif
 
 #ifdef __powerpc64__