[scudo] Enable MemMapFuchsia

[fabio-d]

Summary

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.

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

private:
   uptr Capacity = 0;
   MapPlatformData Data = {};
 };
 
 #if SCUDO_LINUX
 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;
 using MemMapT = ReservedMemoryT::MemMapT;
 #else
 #error                                                                         \
     "Unsupported platform, please implement the ReservedMemory for your platform!"
 #endif
 
 } // namespace scudo
 
 #endif // SCUDO_MEM_MAP_H_