[asan][Darwin] Use Apples blocks extension only when supported (#72639) (#72642)

Issue #72639

The commit at 020cdaff

 broke build of asan on macOS with GCC. GCC does
not support the Apple blocks extension (yet). Uses of blocks in other
parts of the sanitisers are protected by MISSING_BLOCKS_SUPPORT. But the
type definition is not.

_This applies FX's patch from the issue._

Co-authored-by: FX Coudert <fxcoudert@gmail.com>