[GHC] #12878: Use gold linker by default if available on ELF systems

[GHC]

#12878: Use gold linker by default if available on ELF systems
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           Reporter:  dobenour       |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Driver         |           Version:  8.0.1
           Keywords:                 |  Operating System:  Linux
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 `ld.gold` is both faster and less buggy than `ld.bfd`, but only works on
 ELF.  On ELF systems, we should thus prefer `ld.gold` when it is
 installed, even when `ld.bfd` is supported (as it is on x86).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12878>
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