[scudo] Adding an interface function to print allocator stats

Summary:
This adds `__scudo_print_stats` as an interface function to display the Primary
and Secondary allocator statistics for Scudo.

Reviewers: alekseyshl, flowerhack

Reviewed By: alekseyshl

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D46016

llvm-svn: 330857