util: Fix disk_cache index calculation on big endian

util: Fix disk_cache index calculation on big endian

The cache-test test program attempts to create a collision (using key_a
and key_a_collide) by making the first two bytes identical. The idea is
fine -- the shader cache wants to use the first four characters of a
SHA1 hex digest as the index.

The following program

        unsigned char array[4] = {1, 2, 3, 4};
        int *ptr = (int *)array;

        for (int i = 0; i < 4; i++) {
            printf("%02x", array[i]);
        }
        printf("\n");

        printf("%08x\n", *ptr);

prints

   01020304
   04030201

on little endian, and

   01020304
   01020304

on big endian.

On big endian platforms reading the character array back as an int (as
is done in disk_cache.c) does not yield the same results as reading the
byte array.

To get the first four characters of the SHA1 hex digest when we mask
with CACHE_INDEX_KEY_MASK, we need to byte swap the int on big endian
platforms.

Bugzilla: https://bugs.freedesktop.org/103668
Bugzilla: https://bugs.gentoo.org/637060
Bugzilla: https://bugs.gentoo.org/636326
Fixes: 87ab26b2ab35 ("glsl: Add initial functions to implement an
                      on-disk cache")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>