BUILD: ist: do not put a cast in an array declaration

BUILD: ist: do not put a cast in an array declaration

TCC is upset by the declaration looking like:

  const unsigned char ist_lc[256] __attribute__((weak)) = ((const unsigned char[256]){ ... });

It was written like this because it's expanded from the _IST_LC macro
but it's never used as-is, it's only used from ist_lc, which should be
the one containing the cast so that the macro only contains the list of
bytes that can be used in both places. And this assigns more consistent
roles to the lower and upper case macro/variable now, one is typed and
the other one not. No backport is needed.