Add --force-gc support #764

Job	Run time
Test Browser on macOS (safari, disabled)	1m 39s
Test Build	25s
Test Browser on Linux (chrome, default)	1m 2s
Test Browser on Linux (firefox, main)	1m 3s
Test Browser on Linux (chrome, disabled)	55s
Test Browser on macOS (safari, main)	1m 13s
Test Browser on Linux (firefox, disabled)	1m 17s
Test Browser on macOS (safari, default)	2m 20s
Test Browser on Linux (chrome, main)	30s
Test Shell (jsc, main)	43s
Test Shell (spidermonkey, main)	40s
Test Shell (spidermonkey, disabled)	2m 2s
Test Shell (jsc, disabled)	1m 45s
Test Browser on Linux (firefox, default)	1m 17s
Test Shell (spidermonkey, default)	5m 17s
Test Shell (v8, main)	38s
Test Shell (jsc, default)	3m 13s
Test Shell (v8, default)	2m 41s
Test Shell (v8, disabled)	1m 30s
	30m 10s

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