[scudo][standalone] Make malloc_info return a minimal XML

Summary:
Initially, our malloc_info was returning ENOTSUP, but Android would
rather have it return successfully and write a barebone XML to the
stream, so we will oblige.

Add an associated test.

Reviewers: cferris, morehouse, hctim, eugenis, vitalybuka

Reviewed By: morehouse

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D68427

llvm-svn: 373754