Adjust cross-platform code for new disownVM/ownVM API. #387

Job	Run time
squeak.cog.spur for win64x64	3m 4s
squeak.cog.spur for win64x64 (DEBUG)	1m 50s
squeak.cog.spur for win64x64 (ASSERT)	3m 14s
squeak.stack.spur for win64x64	2m 26s
squeak.stack.spur for win64x64 (DEBUG)	2m 36s
squeak.stack.spur for win64x64 (ASSERT)	3m 10s
squeak.sista.spur for win64x64	3m 52s
squeak.sista.spur for win64x64 (DEBUG)	2m 12s
squeak.sista.spur for win64x64 (ASSERT)	3m 44s
squeak.cog.spur for win32x86	2m 56s
squeak.cog.spur for win32x86 (DEBUG)	3m 23s
squeak.cog.spur for win32x86 (ASSERT)	2m 43s
squeak.stack.spur for win32x86	2m 16s
squeak.stack.spur for win32x86 (DEBUG)	1m 50s
squeak.stack.spur for win32x86 (ASSERT)	2m 58s
squeak.sista.spur for win32x86	3m 17s
squeak.sista.spur for win32x86 (DEBUG)	2m 24s
squeak.sista.spur for win32x86 (ASSERT)	3m 29s
	51m 24s

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