[sanitizer] Switch StackStore to 8 MiB blocks

Larger blocks are more convenient for compressions.
Blocks are allocated with MmapNoReserveOrDie to save some memory.

Also it's 15% faster on StackDepotBenchmarkSuite

Depends on D114464.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D114488