Fix alignment of buffers in kernighan_ritchie unittests.

[JohanEngelen]

This fixes a bug with strong optimization (LDC LTO). ubyte arrays are not necessarily word aligned.

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Testing this PR locally

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[JohanEngelen]

I don't understand the circle-CI failure:

parsing std/experimental/allocator/building_blocks/kernighan_ritchie.d
generated/linux/debug/64/publictests/std_experimental_allocator_building_blocks_kernighan_ritchie.d(650): Error: no property `Node` for type `std.experimental.allocator.building_blocks.kernighan_ritchie.KRRegion!(NullAllocator)`
std/experimental/allocator/building_blocks/kernighan_ritchie.d(99):        struct `KRRegion` defined here
make: *** [Makefile:679: std/experimental/allocator/building_blocks/kernighan_ritchie.publictests] Error 1

perhaps something buggy with generating the std_experimental_allocator_building_blocks_kernighan_ritchie.d file?

[kinke]

Please target stable.

Edit: Btw, this was already a problem 7 years ago in #5345. ;)