defaults.h: Default MAX_FIXED_MODE_SIZE to MAX (BITS_PER_WORD * 2, 64)

defaults.h: Default MAX_FIXED_MODE_SIZE to MAX (BITS_PER_WORD * 2, 64)

The old GET_MODE_SIZE (DImode) (i.e. 64) made sense before
64-bitters.  Now the default is just a trap: when using the
default 64, things like TImode (128 bits) still mostly works,
but general corner cases related to computing large-size objects
numbers, like (1 << 64)/8 break, as exposed by
gcc.dg/pr105094.c.

So, keep the floor at 64 for 32-bitters and smaller targets, but
for larger targets, make it 2 * BITS_PER_WORD.  Also, express it
more directly with focus on BITS_PER_WORD, not the size of a
mode.  Add "by GCC internally" in an attempt to tell that this
is when gcc cooks something up, not when plain input uses a type
with such a mode.

	* defaults.h (MAX_FIXED_MODE_SIZE): Default to 2 * BITS_PER_WORD
	for larger-than-32-bitters.
	* doc/tm.texi.in (MAX_FIXED_MODE_SIZE): Adjust accordingly.  Tweak
	wording.
	* doc/tm.texi: Regenerate.