cmake/.../FindSanitizers: add check for Sanitizers_FIBER_SUPPPORT

cmake/.../FindSanitizers: add check for Sanitizers_FIBER_SUPPPORT

With newer clang and gcc versions (observed on clang-17.0.6 as
well as gcc 12/13), asan is throwing stack-use-after-return
during OSD startup related to usage of seastar::async, which
relies on swapcontext internally.

seastar/src/core/thread.cc supports asan's hooks, but only if
SEASTAR_HAVE_ASAN_FIBER_SUPPORT is set.  seastar's CMakeList.txt
sets it based on Sanitizers_FIBER_SUPPORT, which probably should
be set by the module at src/seastar/cmake/FindSanitizers.cmake,
but that module doesn't seem to be actually invoked anywhere.

Ceph's version of that module (cmake/modules/FindSanitizers.cmake)
does not set Sanitizers_FIBER_SUPPORT.

This commit adds that check as well as the related code snippet.

Fixes: https://tracker.ceph.com/issues/64512
Signed-off-by: Samuel Just <sjust@redhat.com>