This re-applies D5195: Add a RTS option -xp to load PIC object anywhere in address space and D5235: Fix T15729 and T3001-2 in master, they were reverted as part of diff
stack to unbreak i386. The proper fix is done in D5289: Add a RTS option -xp to load PIC object anywhere in address space.
Allocate bss section within proper range of other sections:
- when +RTS -xp is passed, allocate it contiguously as we did for jump islands
- when we mmap the code to lower 2Gb, we should allocate bss section there too