rts/Linker: Ensure that mmap_32bit_base is updated after mapping
The amount of duplicated code in mmapForLinker
hid the fact that some
codepaths would fail to update mmap_32bit_base
(specifically, on
platforms like OpenBSD where MAP_32BIT
is not supported). Refactor and simplify the function to make it more obviously correct.
Closes #20734 (closed).