rts/Linker.c: declare 'deRefStablePtr' as an exported 'rts' symbol

    $ inplace/bin/ghc-stage2 -fforce-recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-db -rtsopts -optc-fno-builtin -fno-ghci-history \
        testsuite/tests/ffi/should_run/T4038.hs --interactive -v0 -ignore-dot-ghci +RTS -I0.1 -RTS

    *Main> main
    <interactive>: /tmp/ghc16668_0/ghc16668_5.o: unknown symbol `deRefStablePtr'

The reference to 'deRefStablePtr' is generated by 'compiler/deSugar/DsForeign.lhs':
    the_cfun
       = case maybe_target of
          Nothing    -> text "(StgClosure*)deRefStablePtr(the_stableptr)"
          Just hs_fn -> char '&' <> ppr hs_fn <> text "_closure"

Patch fixes all broken tests using 'import wrapper':
    TEST="ffi013 ffi010 ffi011 ffi005 ffi020 ffi006 ffi019 fed001 T1679 T4038"

Tests manifested as broken only in DYNAMIC_GHC_PROGRAMS=NO builds,
where GHCi's custom linker is used instead of system's linker.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>