overflow: Introduce wrapping_add(), wrapping_sub(), and wrapping_mul()

Provide helpers that will perform wrapping addition, subtraction, or
multiplication without tripping the arithmetic wrap-around sanitizers. The
first argument is the type under which the wrap-around should happen
with. In other words, these two calls will get very different results:

	wrapping_mul(int, 50, 50) == 2500
	wrapping_mul(u8,  50, 50) ==  196

Add to the selftests to validate behavior and lack of side-effects.

Reviewed-by: Gustavo A. R. Silva <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Acked-by: Mark Rutland <[email protected]>
Signed-off-by: Kees Cook <[email protected]>

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