This adds an interface for a program to iterate over all Fake stack regions.
In particular, this enables a garbage collector to correctly observe reference to objects that are on the Fake stack (the normal stack would be scanned, but the Fake stack cannot be scanned without this added interface).
Two functions are added. One to scan only the provided thread ID's Fake stack, and one to scan all Fake stacks for all threads known to ASan.
Currently no test case is included. Please advice on how best to test this and in which file to place the testcases.