kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE

Patch series "Fix some incompatibilites between KASAN and FORTIFY_SOURCE", v4.

3 KASAN self-tests fail on a kernel with both KASAN and FORTIFY_SOURCE:
memchr, memcmp and strlen.

When FORTIFY_SOURCE is on, a number of functions are replaced with
fortified versions, which attempt to check the sizes of the operands.
However, these functions often directly invoke __builtin_foo() once they
have performed the fortify check.  The compiler can detect that the
results of these functions are not used, and knows that they have no other
side effects, and so can eliminate them as dead code.

Why are only memchr, memcmp and strlen affected?
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Of string and string-like functions, kasan_test tests:

 * strchr  ->  not affected, no fortified version
 * strrchr ->  likewise
 * strcmp  ->  likewise
 * strncmp ->  likewise

 * strnlen ->  not affected, the fortify source implementation calls the
             ...