Singleton classes of unshareable objects are not shareable #12577

Job	Run time
ruby-bench (--warmup=1 --bench=1 --excludes=lobsters)	16m 38s
make (check, i686)	11m 45s
make (test-bundler-parallel, 50)	24m 31s
make (check, cppflags=-DVM_CHECK_MODE)	9m 4s
make (check, ubuntu-24.04, capi)	10m 8s
make (check, --disable-yjit)	7m 53s
make (check, --enable-shared --enable-load-relative)	8m 49s
make-ibm	-1s
make (test-bundled-gems)	19m 21s
Ubuntu result	2s
	1h 48m 10s

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