[scudo] Separate Fuchsia & Default SizeClassMap

The Fuchsia allocator config was using the default size class map.

This CL gives Fuchsia its own size class map and changes a couple of
things in the default one:
- make `SizeDelta` configurable in `Config` for a fixed size class map
  as it currently is for a table size class map;
- switch `SizeDelta` to 0 for the default config, it allows for size
  classes that allow for power of 2s, and overall better wrt pages
  filling;
- increase the max number of caches pointers to 14 in the default,
  this makes the transfer batch 64/128 bytes on 32/64-bit platforms,
  which is cache-line friendly (previous size was 48/96 bytes).

The Fuchsia size class map remains untouched for now, this doesn't
impact Android which uses the table size class map.

Differential Revision: https://reviews.llvm.org/D102783