[GHC] #8701: Update libffi-tarballs to latest libffi

[GHC]

#8701: Update libffi-tarballs to latest libffi
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        Reporter:  lukexi            |            Owner:
            Type:  bug               |           Status:  new
        Priority:  high              |        Milestone:  7.8.1
       Component:  Compiler (FFI)    |          Version:  7.8.1-rc1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  Building GHC      |       Difficulty:  Easy (less than 1
  failed                             |  hour)
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by hvr):

 Why isn't there an official `libffi-3.0.14.tar.gz` at
 https://sourceware.org/libffi/?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8701#comment:1>
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