[scudo] Change configuration for Trusty, use mmap()

Trusty runs in memory constrained environments, with many apps
having only one page (4KB) of heap memory available. However, we
still want to mmap() multiples of PAGE_SIZE at a time.

Additionally, switch Scudo from using sbrk() to mmap().

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D151968