gcc: quiet Wattribute for no_sanitize("address")

This is an unfortunate instance where the __has_attribute check does
not function usefully. Gcc does have the attribute, but for gcc it only
applies to functions, not variables, and trying to apply it to a
variable generates Wattribute. So far we only apply the attribute to
variables. Only enable the attribute for clang, for now.

Reviewed by: Anton Rang <rang at acm.org>
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22875