rts: make it possible to change mblock size on 32-bit targets

The MBLOCK_SHIFT macro must be the single source of truth for defining the mblock size, and changing it should only affect performance, not correctness. This patch makes it truly possible to reconfigure mblock size, at least on 32-bit targets, by fixing places which implicitly relied on the previous MBLOCK_SHIFT constant. Fixes #22901 (closed).