[PATCH] correct an ILP32/LP64 bug in sprintf warning (PR 91567)

[Martin Sebor]

The recent sprintf+strlen integration doesn't handle unbounded
string lengths entirely correctly for ILP32 targets and causes
-Wformat-overflow false positives in some common cases, including
during GCC bootstrap targeting such systems  The attached patch
fixes that mistake.  (I think this code could be cleaned up and
simplified some more but in the interest of unblocking the ILP32
bootstrap and Glibc builds I haven't taken the time to do that.)
The patch also adjusts down the maximum strlen result set by EVRP
to PTRDIFF_MAX - 2, to match what the strlen pass does.

The strlen maximum would ideally be computed in terms of
max_object_size() (for which there would ideally be a --param
setting), and checked the same way to avoid off-by-one mistakes
between subsystems and their clients.  I have not made this change
here but added a FIXME comment mentioning it.  I plan to add such
a parameter and use it in max_object_size() in a future change.

Testing with an ILP32 compiler also ran into the known limitation
of the strlen pass being unable to determine the length of array
members of local aggregates (PR 83543) initialized using
the braced-list syntax.  gcc.dg/tree-ssa/builtin-snprintf-6.c
fails a few cases as a result.    I've xfailed the assertions
for targets other than x86_64 where it isn't an issue.

Martin
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