Implement variant of crucible_alloc with user-specified alignment

[brianhuffman]

In a CrucibleSetup block, crucible_alloc always chooses an alignment based on the maximum required for any primitive type (usually 8 bytes).

saw-script/src/SAWScript/Crucible/LLVM/Builtins.hs

Lines 679 to 691 in 15d2548

	doAlloc ::
	(?lc :: Crucible.TypeContext, Crucible.HasPtrWidth (Crucible.ArchWidth arch)) =>
	LLVMCrucibleContext arch ->
	LLVMAllocSpec ->
	StateT MemImpl IO (LLVMPtr (Crucible.ArchWidth arch))
	doAlloc cc (LLVMAllocSpec mut _memTy sz loc) = StateT $ \mem ->
	do let sym = cc^.ccBackend
	let dl = Crucible.llvmDataLayout ?lc
	sz' <- W4.bvLit sym Crucible.PtrWidth $ Crucible.bytesToInteger sz
	let alignment = Crucible.maxAlignment dl -- Use the maximum alignment required for any primitive type (FIXME?)
	let l = show (W4.plSourceLoc loc)
	liftIO $
	Crucible.doMalloc sym Crucible.HeapAlloc mut l mem sz' alignment

However, sometimes C/LLVM functions might require more-aligned pointers. E.g. if a type is declared with __attribute__((aligned(64))), then dereferencing a pointer to it would require 64-byte alignment. (Thanks to @andreistefanescu for bringing this to my attention.)

To support specifying and verifying such code, we should implement a variant of crucible_alloc called something like crucible_alloc_aligned that takes an extra numeric argument for the desired alignment. This should be pretty easy to implement, as the Crucible/LLVM memory model allows arbitrary alignments to be specified for each allocation.

[brianhuffman]

Implemented by PR #637.