[scudo][standalone] Restore GWP-ASan flag parsing

With D92696, the Scudo Standalone GWP-ASan flag parsing was changed to
the new GWP-ASan optional one. We do not necessarily want this, as this
duplicates flag parsing code in Scudo Standalone when using the
GWP-ASan integration.

This CL reverts the changes within Scudo Standalone, and increases
`MaxFlags` to 20 as an addionnal option got us to the current max.

Differential Revision: https://reviews.llvm.org/D95542