Improve error message when mixing 32bit and 64bit images

Since we know when the user has mixed 32 and 64 bit images, let's say so in the error message. Before:

Loading object (static) dist/build/HOC_cbits.o ... ghc: dist/build/HOC_cbits.o: Bad magic. Expected: feedfacf, got: feedface.

ghc: panic! (the 'impossible' happened)
  (GHC version 7.6.1 for x86_64-apple-darwin):
        loadObj "dist/build/HOC_cbits.o": failed

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

After:

Loading object (static) dist/build/HOC_cbits.o ... ghc-stage2: Could not load image dist/build/HOC_cbits.o: bad magic!
  Expected feedfacf (64bit), got feedface (32bit).

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.7.20130124 for x86_64-apple-darwin):
        loadObj "dist/build/HOC_cbits.o": failed

Patch to come.

Trac metadata

Trac field	Value
Version	7.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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