[scudo] SCUDO_FUCHSIA uses ReservedMemoryDefault

Some Fuchsia zx tests failed from https://reviews.llvm.org/D153888:
https://turquoise-internal-review.git.corp.google.com/c/integration/+/729619

Use `ReservedMemoryDefault` for `SCUDO_FUCHSIA` to use the default
MemMap API, while test failures are debugged.

Differential Revision: https://reviews.llvm.org/D154538