wide-int: Add simple CHECKING_P stack-protector canary like checking

This patch adds hopefully not so expensive --enable-checking=yes
verification that the widest_int upper length bound estimates are really
upper bounds and nothing attempts to write more elements.
It is done only if the estimated upper length bound is smaller than
WIDE_INT_MAX_INL_ELTS, but that should be the most common case unless
large _BitInt is involved.

2023-10-12  Jakub Jelinek  <jakub@redhat.com>

	* wide-int.h (widest_int_storage <N>::write_val): If l is small
	and there is space in u.val array, store a canary value at the
	end when checking.
	(widest_int_storage <N>::set_len): Check the canary hasn't been
	overwritten.