Add a RTS option -xp to load PIC object anywhere in address space

This stacks three accepted revisions in Phabricator:

• https://phabricator.haskell.org/D5289 Add a RTS option -xp to load PIC object anywhere in address space
• https://phabricator.haskell.org/D5290 Allocate bss section within proper range of other sections
• https://phabricator.haskell.org/D5233 Don't use X86_64_ELF_NONPIC_HACK for +RTS -xp

Rebase and port them here and make sure that CI passed.