Problem in C Code Generator, with patch

[Christopher Vollick]

Hey guys!
So, I was working on some cross-compiling, and found a bug
(http://hackage.haskell.org/trac/ghc/ticket/7620)

Eventually I found that the issue was with C CodeGen in general, and had
nothing to do with ARM.

I've attached the patch there, but it may be only part of a bigger
problem.

The rough problem came from two things:
1) -(-128 :: Int8) => -128
	You can't negate the smallest int in an integer space because of how
	2s compliment works. There aren't enough bits to store +128.

2) We have a number of operations that transform:
	| i < 0 = a + i => a - (negative i)
	And
	| i < 0 = a - i => a + (negative i)

And the assumption is that any integer can be negated, but that's not
true in exactly one case.

So, for example, it could turn
	thing+-128
into
	thing--128
Which is not valid C.
That (but for Int32) was what I was seeing, and causing me trouble
building integer simple.

So, that was the bug I had encountered, and I don't know how the GHC
code works, I just trawled through until I found the line that was
causing me trouble, but I just figured I'd remind people that not every
integer can be negated, in case the same mistake is being done in other
places.

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