Adjusted macros to avoid working outside OC_IDCT_CONSTS array.

The OC_IDCT_CONSTS array in x86/sse2idct.c is 64 elements long, so
not use trying to pretend it is 128 elements long.

This avoid the following copmiler message:

In function 'oc_idct8x8_slow_mmx',
    inlined from 'oc_idct8x8_mmx' at ../../lib/x86/mmxidct.c:555:8:
../../lib/x86/mmxidct.c:290:3: warning: array subscript 'const struct <anonymous>[0]' is partly outside array bounds of 'const short unsigned int[64]' [-Warray-bounds]
  290 |   __asm__ __volatile__(
      |   ^~~~~~~
In file included from ../../lib/x86/mmxidct.c:20:
../../lib/x86/x86int.h: In function 'oc_idct8x8_mmx':
../../lib/x86/x86int.h:95:58: note: object 'OC_IDCT_CONSTS' of size 128
   95 | extern const unsigned short __attribute__((aligned(16))) OC_IDCT_CONSTS[64];
      |                                                          ^~~~~~~~~~~~~~
In function 'oc_idct8x8_10_mmx',
    inlined from 'oc_idct8x8_mmx' at ../../lib/x86/mmxidct.c:554:20:
../../lib/x86/mmxidct.c:492:3: warning: array subscript 'const struct <anonymous>[0]' is partly outside array bounds of 'const short unsigned int[64]' [-Warray-bounds]
  492 |   __asm__ __volatile__(
      |   ^~~~~~~
../../lib/x86/x86int.h: In function 'oc_idct8x8_mmx':
../../lib/x86/x86int.h:95:58: note: object 'OC_IDCT_CONSTS' of size 128
   95 | extern const unsigned short __attribute__((aligned(16))) OC_IDCT_CONSTS[64];
      |                                                          ^~~~~~~~~~~~~~
  CC       x86/sse2idct.lo
In function 'oc_idct8x8_slow_sse2',
    inlined from 'oc_idct8x8_sse2' at ../../lib/x86/sse2idct.c:453:8:
../../lib/x86/sse2idct.c:213:3: warning: array subscript 'const struct <anonymous>[0]' is partly outside array bounds of 'const short unsigned int[64]' [-Warray-bounds]
  213 |   __asm__ __volatile__(
      |   ^~~~~~~
../../lib/x86/sse2idct.c: In function 'oc_idct8x8_sse2':
../../lib/x86/sse2idct.c:26:56: note: object 'OC_IDCT_CONSTS' of size 128
   26 | const unsigned short __attribute__((aligned(16),used)) OC_IDCT_CONSTS[64]={
      |                                                        ^~~~~~~~~~~~~~
In function 'oc_idct8x8_10_sse2',
    inlined from 'oc_idct8x8_sse2' at ../../lib/x86/sse2idct.c:452:20:
../../lib/x86/sse2idct.c:398:3: warning: array subscript 'const struct <anonymous>[0]' is partly outside array bounds of 'const short unsigned int[64]' [-Warray-bounds]
  398 |   __asm__ __volatile__(
      |   ^~~~~~~
../../lib/x86/sse2idct.c: In function 'oc_idct8x8_sse2':
../../lib/x86/sse2idct.c:26:56: note: object 'OC_IDCT_CONSTS' of size 128
   26 | const unsigned short __attribute__((aligned(16),used)) OC_IDCT_CONSTS[64]={
      |                                                        ^~~~~~~~~~~~~~