[scudo] Enable Scudo memory hooks for Fuchsia.

Summary:
    It would be useful for Flutter apps, especially, to be able to use
    malloc hooks to debug memory leaks on Fuchsia. They're not able to do
    this right now, so it'd be a nice bonus to throw in with the Scudo
    switchover.


Reviewers: cryptoad, alekseyshl

Reviewed By: cryptoad

Differential Revision: https://reviews.llvm.org/D48618

llvm-svn: 336139