tsan: fix broken aarch64_39/42 mappings and expand them

The aarch64 39- and 42-bit mappings were broken: mappings to meta and shadow were not fully invertible. This CL introduces a working set of mappings, and also increases the size of some app regions:
* aarch64, 39-bit (2^39 == 512GB):
 - Low: (Old) 4GB -> (New) 20GB
 - Mid: 4GB -> 20GB
 - Heap: 4GB -> 12GB
 - High: 8GB -> 12GB
* aarch64, 42-bit (2^42 == 4TB):
 - Low: 64GB -> 128GB
 - Mid: 4GB -> 88GB
 - Heap: 64GB -> 192GB
 - High: 64GB

Additionally, this CL improves the code comments for all the linux aarch64 mappings.

Differential Revision: https://reviews.llvm.org/D141640