Don't use '-z defs' with Clang's sanitizers

The clang documentation in all sanitizers we currently use says this:

    When linking shared libraries, the {flavor}Sanitizer run-time is
    not linked, so -Wl,-z,defs may cause link errors (don’t use it
    with {flavor}Sanitizer)

(in our case, {flavor} is one of Address, Memory, or UndefinedBehavior)

Therefore, we turn off that particular flag specifically when using
the sanitizers.

Fixes #8735

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8749)