fiber: disable fiber stack protection with ASAN temporarily

If leak sanitizer reaches the memory protected from read with mprotect
it exhibits all sorts of odd behaviour. It can hang, can crash, can
return errors with no leak backtraces.

We use mprotect to create guard zones at the end of fiber stack so if
stack is overflowed we get a signal and crash. We take protection off
when fiber is destroyed. Unfortunately we do not destroy cords (and its
fibers) which cancelled through cord_cancel_and_join. This is going to
be addressed in patch for issue #8423 ("Get rid of pthread_cancel()").
Until that moment let's disable protection for ASAN builds.

Note that we did not hit this behaviour before because LSAN only scans
memory allocated using malloc and regular slab cache uses mmap to get
memory.

Part of #7327

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(cherry picked from commit 2ee15793)