macro: add macro for llvm no_sanitize_address attribute

We want it for global variables, which LLVM supports and GCC currently
does not (GCC does support it for functions, but we care about global
variables here).

Why is this relevant? When asan is used global variables are padded with
hotzones before and after. But we can't have that for the registration
variables we place in special ELF sections: we want them tightly packed
so that we can iterate through them.

Note that for gcc this isn't an issue, as it will pack stuff in
non-standard sections anyway, even if asan is used.