CMM: add a mechanism to import C .data labels

Summary:
This introduces new .cmm syntax for import:

    'import' 'CLOSURE' <identifier>;

Currently cmm syntax allows importing only function labels:

    import pthread_mutex_lock;

but sometimes ghc needs to import global gariables
or haskell closures:

    import ghczmprim_GHCziTypes_True_closure;
    import base_ControlziExceptionziBase_nestedAtomically_closure;
    import ghczmprim_GHCziTypes_False_closure;
    import sm_mutex;

It breaks on ia64 where there is a difference in
pointers to data and pointer to functions.

Patch fixes threaded runtime on ia64 where
dereference of 'sm_mutex' from CMM led to
incurrect location.

Exact breakage machanics are the same as in e18525fae273f4c1ad8d6cbe1dea4fc074cac721

Merge into the 7.10 branch

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

Test Plan: passes ./validate, makes ghci work on ghc-7.8.4

Reviewers: simonmar, simonpj, austin

Reviewed By: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D622

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