[scudo][standalone] Allow fallback to secondary if primary is full

Summary:
We introduced a way to fallback to the immediately larger size class for
the Primary in the event a region was full, but in the event of the largest
size class, we would just fail.

This change allows to fallback to the Secondary when the last region of
the Primary is full. We also expand the trick to all platforms as opposed
to being Android only, and update the test to cover the new case.

Reviewers: hctim, cferris, eugenis, morehouse, pcc

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D76430