cmake: extract sanitizer detection out

cmake: extract sanitizer detection out

also, we don't need to pass '-lasan' or '-ltsan' to linker. it's
suggested to use the "-fsanitize=${sanitizer}".

please note, this module is compatible with the one used in seastar on
purpose, as seastar is included in ceph using add_subdirectory(), and it
in turn add its own cmake modules directory using list(APPEND ...), so
cmake/modules/FindSantitizers.cmake is prefered over
src/seastar/cmake/FindSantitizers.cmake.

Signed-off-by: Kefu Chai <kchai@redhat.com>