[GHC] #8446: iOS/Cross-compilation: Add build.mk.sample entry

[GHC]

#8446: iOS/Cross-compilation: Add build.mk.sample entry
------------------------------------+-------------------------------------
       Reporter:  lukexi            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  high              |         Milestone:  7.8.1
      Component:  Build System      |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 Adds an entry to build.mk.sample configured for cross-compilation (used
 for iOS cross-compilation, but should be generally applicable).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8446>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler