Implement PowerPC 64-bit native code backend for Linux

Extend the PowerPC 32-bit native code generator for "64-bit
PowerPC ELF Application Binary Interface Supplement 1.9" by
Ian Lance Taylor and "Power Architecture 64-Bit ELF V2 ABI Specification --
OpenPOWER ABI for Linux Supplement" by IBM.
The latter ABI is mainly used on POWER7/7+ and POWER8
Linux systems running in little-endian mode. The code generator
supports both static and dynamic linking. PowerPC 64-bit
code for ELF ABI 1.9 and 2 is mostly position independent
anyway, and thus so is all the code emitted by the code
generator. In other words, -fPIC does not make a difference.

rts/stg/SMP.h support is implemented.

Following the spirit of the introductory comment in
PPC/CodeGen.hs, the rest of the code is a straightforward
extension of the 32-bit implementation.

Limitations:
* Code is generated only in the medium code model, which
  is also gcc's default
* Local symbols are not accessed directly, which seems to
  also be the case for 32-bit
* LLVM does not work, but this does not work on 32-bit either
* Must use the system runtime linker in GHCi, because the
  GHC linker for "static" object files (rts/Linker.c) for
  PPC 64-bit is not implemented. The system runtime
  (dynamic) linker works.
* The handling of the system stack (register 1) is not ELF-
  compliant so stack traces break. Instead of allocating a new
  stack frame, spill code should use the "official" spill area
  in the current stack frame and deallocation code should restore
  the back chain
* DWARF support is missing

Fixes #9863

Test Plan: validate (on powerpc, too)

Reviewers: simonmar, trofi, erikd, austin

Reviewed By: trofi

Subscribers: bgamari, arnons1, kgardas, thomie

Differential Revision: https://phabricator.haskell.org/D629

GHC Trac Issues: #9863