[scudo] Fix definition of SCUDO_SMALL_STACK_DEPOT.

The SCUDO_FUZZ macro is either defined or not defined. The previous
code assumed it had a one or zero value, so change the setting of
SCUDO_SMALL_STACK_DEPOT based on defined(SCUDO_FUZZ).

Reviewed By: Chia-hungDuan

Differential Revision: https://reviews.llvm.org/D158459