Linux

Impart more safety to extract-object-from-core #3

Job	Run time
build (--with-sb-thread, x86)	18m 36s
build (--with-sb-thread, x86-64)	17m 9s
build (--without-sb-thread, x86)	19m 2s
build (--without-sb-thread, x86-64)	17m 29s
build (--without-sb-unicode, x86)	17m 32s
build (--without-sb-unicode, x86-64)	17m 46s
build (x86-64, sse4, --with-sb-thread)	30m 15s
build (x86-64, fasteval, --with-sb-fasteval --without-sb-eval)	25m 58s
build (x86-64, --with-mark-region-gc)	19m 42s
	3h 3m 29s

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