[scudo][standalone] Allow setting release to OS

Summary:
Add a method to set the release to OS value as the system runs,
and allow this to be set differently in the primary and the secondary.
Also, add a default value to use for primary and secondary. This
allows Android to have a default that is different for
primary/secondary.

Update mallopt to support setting the release to OS value.

Reviewers: pcc, cryptoad

Reviewed By: cryptoad

Subscribers: cryptoad, jfb, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D74448