Add back off strategy for large reservations

[mjp41]

In the POSIX and Windows PALs, if aligned allocation isn't provided we reserve a large amount of memory, so that we can perform alignment work in the AddressSpaceManager. However, if the large reservation fails, then the system will determine out-of-memory.

The PALs should attempt to reduce the size of reservation in the failure case, so that the program can continue working if possible.

[SchrodingerZhu]

I see. I mentioned this at #223 because it is somehow related to the allocation overhead, but now I can understand the true reason is behind this reservation behavior.

[mjp41]

@SchrodingerZhu thanks for your investigation into the other issue, and report this finding. It is important to get this right.