fill_alignment_gap potentially produces UB

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In the function here: https://github.com/mmtk/mmtk-core/blob/master/src/util/alloc/allocator.rs#L90 the assumption is that ALIGNMENT_VALUE fits into BYTES_IN_INT, but BYTES_IN_INT is 4 everywhere while ALIGNMENT_VALUE is defined as usize. On top of that it does store(usize) so it will write 8 bytes on 64-bit targets even if ALIGNMENT_VALUE fits into 32 bits.