Run GC if fiber pool expansion fails. (#16535) #12801

Job	Run time
ruby-bench (--warmup=1 --bench=1)	16m 28s
make (check, cppflags=-DVM_CHECK_MODE)	9m 7s
make (check, --enable-shared --enable-load-relative)	7m 58s
make (test-bundler-parallel, 50)	23m 54s
make (test-bundled-gems)	20m 33s
make (check, ubuntu-24.04, capi)	10m 39s
make (check, --disable-yjit)	7m 43s
make (check, i686)	12m 6s
make-ibm	-1s
Ubuntu result	4s
	1h 48m 31s

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