Zejun Wu authored Oct 15, 2018 and Ben Gamari committed Oct 30, 2018

Add a RTS option -xp to load PIC object anywhere in address space. We do
this by relaxing the requirement of <0x80000000 result of
`mmapForLinker` and implying USE_CONTIGUOUS_MMAP.

We also need to change calls to `ocInit` and `ocGetNames` to avoid
dangling pointers when the address of `oc->image` is changed by
`ocAllocateSymbolExtra`.

Test Plan:
```
$ uname -a
Linux localhost 4.18.8-arch1-1-ARCH #1 SMP PREEMPT Sat Sep 15 20:34:48
UTC 2018 x86_64 GNU/Linux
$ cat mk/build.mk
DYNAMIC_GHC_PROGRAMS = NO
DYNAMIC_BY_DEFAULT = NO

GhcRTSWays += thr_debug
EXTRA_HC_OPTS += -debug
WAY_p_HC_OPTS += -fPIC -fexternal-dynamic-refs
$ inplace/bin/ghc-stage2 --interactive -prof +RTS -xp
GHCi, version 8.7.20180928: http://www.haskell.org/ghc/  :? for help
ghc-stage2: R_X86_64_32 relocation out of range:
ghczmprim_GHCziTypes_ZMZN_closure = 7f690bffab59
Recompile
/data/users/watashi/ghc/libraries/ghc-prim/dist-install/build/HSghc-prim
-0.5.3.o with -fPIC -fex...