[scudo] Tune FuchsiaConfig for RiscV

Reduce `PrimaryRegionSizeLog` to 28U to be compatible with a 38bit user
address space on the fuchsia-riscv platform. `PrimaryGroupSizeLog` is
reduced to 19 to preserve 512 BatchGroups per region.

This change can be tested on Fuchsia with:

```
fx set --auto-dir bringup.riscv64 --with //bundles:boot_tests \
fx build bundles:boot_tests \
fx run-boot-test --args={-s,1} boot-libc-unittests \
--cmdline='--gtest_filter=-*DeathTest*:PthreadGetSet*:ScudoSecondaryTest*'
```
The gtest filter ignores pthread and death tests due to non-scudo
related issues on fuchsia-riscv (ScudoSecondaryTest includes a death
check).

Related Ticket: https://fxbug.dev/125263

Differential Revision: https://reviews.llvm.org/D148475