Add alignment-restricted ForeignPtr allocation actions

Whilst GHC.ForeignPtr exports some actions to allocate buffers containing an element of some type, or of some specific byte-length efficiently, and there's an implementation of newAlignedPinnedByteArray# used internally, there's no way to request such buffer of a given size and at a given alignment.

The ability to enforce a specific alignment for some memory buffer is important e.g. when working with SIMD instructions (which is my original use-case). My attempts to work-around the lack of this function failed miserably (with a reproducible test-case) as mentioned in [1].

[1] http://www.haskell.org/pipermail/glasgow-haskell-users/2012-July/022579.html

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Trac field	Value
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Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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