This commit makes overflow of signed integers to behave as expected by at 90%
of the C developers, even though overflow of signed integers are strictly
undefined behavior.
Note: Please do not add code relying on a specific behavior for the overflow of
signed integers, even though `-fwrpav` will make that code work. This is
intended to mitigate the risk of bugs in overflow checks being exploited,
not to encourage adding new bugs.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30475 for details and see
http://c-faq.com/misc/intovf.html on how to implement overflow checks properly.
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