[GHC] #11578: Fix GHC on AArch64/Arm64

[GHC]

#11578: Fix GHC on AArch64/Arm64
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           Reporter:  erikd     |             Owner:
               Type:  bug       |            Status:  new
           Priority:  normal    |         Milestone:  8.2.1
          Component:  Compiler  |           Version:  8.1
           Keywords:            |  Operating System:  Unknown/Multiple
       Architecture:  aarch64   |   Type of failure:  None/Unknown
          Test Case:            |        Blocked By:
           Blocking:            |   Related Tickets:
Differential Rev(s):            |         Wiki Page:
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 Raising this to track discussion about GHC on Arm64. There is already some
 discussion in ticket #11206 (ARM is generally a disaster), but that ticket
 is about 32 bit Arm which is vastly different and is now mostly fixed.

 Information worth saving from #11206:

 * GHC 7.8.4 from Debian can build the ghc-8.0 branch, but cannot build
 HEAD because HEAD requires ghc-7.10 to build.
 * GHC 7.10 from Debian is broken because of an unfixed bug in llvm-3.5.2.
 * User @mfox reports that ghc-7.10 does build against llvm-3.7 and results
 in a compiler that can build HEAD.
 * That GHC 7.10 can be build against llvm-3.7 is a happy accident.
 Versions of GHC are usually locked to a specific version of LLVM.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11578>
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