mmix: Define MAX_FIXED_MODE_SIZE

mmix: Define MAX_FIXED_MODE_SIZE

Besides this commit working as a release-branch fix for the
PR, code inspection shows slightly better code for TImode
libgcc functions, and a modified
gcc.c-torture/execute/arith-rand-ll.c (basically s/long
long/__int128 and cutting out the non-128-bit cases) shows a
1.4% improvement.  (Coremark code is identical, as
expected.)

	PR middle-end/120935
	* config/mmix/mmix.h (MAX_FIXED_MODE_SIZE): Define.

Co-authored-by: Pietro Monteiro <pietro@sociotechnical.xyz>
Signed-off-by: Pietro Monteiro <pietro@sociotechnical.xyz>