[Bug sanitizer/78267] [7 Regression] libsanitizer breaks bootstrap on x86_64-apple-darwin16 at r241977

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78267

--- Comment #29 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Maxim Ostapenko from comment #28)
> So, perhaps I can commit it (from #15) as Jakub suggested to restore GCC
> bootstrap for now?

if we're going down that path can we make the conditional __BLOCKS__ instead of
__clang__ since that's the actual constraint.

FWIW, the SDKs don't generally change once N+1 is out, so I think that the
fixes to 10.10 and 10.11 will not be fragile, they just need doing and testing.