[scudo] Add default implementations for weak functions

Summary:
This is in preparation for platforms where `SANITIZER_SUPPORTS_WEAK_HOOKS` is 0.
They require a default implementation.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D42557

llvm-svn: 323795