[scudo] Enable Scudo on PPC64

Summary:
In conjunction with the clang side change D48833, this will enable Scudo on
PPC64. I tested `check-scudo` on a powerpc64le box and everything passes.

Reviewers: eugenis, alekseyshl

Reviewed By: alekseyshl

Subscribers: mgorny, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D48834

llvm-svn: 336213