[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

Author: fabio-d

compiler-rt/lib/scudo/standalone/mem_map.h

@@ -76,7 +76,7 @@ class ReservedMemoryDefault final
 using ReservedMemoryT = ReservedMemoryDefault;
 using MemMapT = ReservedMemoryT::MemMapT;
 #elif SCUDO_FUCHSIA
-using ReservedMemoryT = ReservedMemoryDefault;
+using ReservedMemoryT = ReservedMemoryFuchsia;
 using MemMapT = ReservedMemoryT::MemMapT;
 #elif SCUDO_TRUSTY
 using ReservedMemoryT = ReservedMemoryDefault;