As done with other image addresses in other architectures, use an
explicit flexible array instead of "address of char", which can trip
bounds checking done by the compiler. Found when building with
-Warray-bounds:

In file included from ./include/linux/byteorder/little_endian.h:5,
                 from ./arch/mips/include/uapi/asm/byteorder.h:15,
                 from ./arch/mips/include/asm/bitops.h:21,
                 from ./include/linux/bitops.h:33,
                 from ./include/linux/kernel.h:22,
                 from arch/mips/boot/compressed/decompress.c:13:
arch/mips/boot/compressed/decompress.c: In function 'decompress_kernel':
./include/asm-generic/unaligned.h:14:8: warning: array subscript -1 is outside array bounds of 'unsigned char[1]' [-Warray-bounds]
   14 |  __pptr->x;        \
      |  ~~~~~~^~~
./include/uapi/linux/byteorder/little_endian.h:35:51: note: in definition of macro '__le32_to_cpu'
   35 | #define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
      |                                                   ^
./include/asm-generic/unaligned.h:32:21: note: in expansion of macro '__get_unaligned_t'
   32 |  return le32_to_cpu(__get_unaligned_t(__le32, p));
      |                     ^~~~~~~~~~~~~~~~~
arch/mips/boot/compressed/decompress.c:29:37: note: while referencing '__image_end'
   29 | extern unsigned char __image_begin, __image_end;
      |                                     ^~~~~~~~~~~

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>