[GHC] #7902: Add support for byte endianness swapping exposed as a primops

[GHC]

#7902: Add support for byte endianness swapping exposed as a primops
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Reporter:  tab               |          Owner:                  
    Type:  feature request   |         Status:  patch           
Priority:  normal            |      Component:  Compiler        
 Version:  7.7               |       Keywords:  bswap endianness
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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Comment(by tab):

 * I started originally with a bSwap16# too, but i removed it to focus on
 the 32 bits version and make sure i'ld plumb it in all the correct place.
 bSwap16# is not very hard indeed and i can re-add it properly quickly now
 i'm a bit more familiar with ghc's codegen.

 * I'm not sure how to expose it to the world, as Bits is quite big and not
 sure what byte endianess reversing mean for signed int and such. So i'll
 be happy to expose it somewhere following some discussion and direction on
 this.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7902#comment:4>
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