[scudo] Enable MemMapFuchsia

This CL re-enables the new MemMapFuchsia backend which was added
in https://reviews.llvm.org/D153888 but then disabled because of
failing tests in https://reviews.llvm.org/D154538.

The failing tests should be fixed now, let's reland it.

Reviewed By: Caslyn

Differential Revision: https://reviews.llvm.org/D156800