util/disk_cache: use rand_xorshift128plus() to get our random int
util/disk_cache: use rand_xorshift128plus() to get our random int

Otherwise for apps that don't seed the regular rand() we will always
remove old cache entries from the same dirs.

V2: assume bits returned by rand are independent uniformly distributed
    bits and grab our hex value without taking the modulus of the whole
    value, this also fixes a bug where 'f' was always missing.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler