powerpc: fix and enable shared libraries by default on linux

Summary:
And fix things all the way down to it. Namely:
    - remove 'r30' from free registers, it's an .LCTOC1 register
      for gcc. generated .plt stubs expect it to be initialised.
    - fix PicBase computation, which originally forgot to use 'tmp'
      reg in 'initializePicBase_ppc.fetchPC'
    - mark 'ForeighTarget's as implicitly using 'PicBase' register
      (see comment for details)
    - add 64-bit MO_Sub and test on alloclimit3/4 regtests
    - fix dynamic label offsets to match with .LCTOC1 offset

Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Test Plan: validate passes equal amount of vanilla/dyn tests

Reviewers: simonmar, erikd, austin

Reviewed By: erikd, austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D560

GHC Trac Issues: #8024, #9831