Trac #9878: Have StaticPointers support dynamic loading.

Summary:
A mutex is used to protect the SPT.

unsafeLookupStaticPtr and staticPtrKeys in GHC.StaticPtr are made
monadic.

SPT entries are removed in a destructor function of modules.

Authored-by: Facundo Domínguez <facundo.dominguez@tweag.io>
Authored-by: Alexander Vershilov <alexander.vershilov@tweag.io>

Test Plan: ./validate

Reviewers: austin, simonpj, hvr

Subscribers: carter, thomie, qnikst, mboes

Differential Revision: https://phabricator.haskell.org/D587

GHC Trac Issues: #9878

(cherry picked from commit 7637810a)