Add major GC reasons to GC.stat #12648

Job	Run time
omnibus compilations, trigger	4s
omnibus compilations, #1	29m 17s
omnibus compilations, #5	27m 30s
omnibus compilations, #2	36m 23s
omnibus compilations, #3	35m 0s
omnibus compilations, #11	23m 4s
omnibus compilations, #8	28m 52s
omnibus compilations, #12	15m 44s
omnibus compilations, #10	30m 10s
omnibus compilations, #7	35m 30s
omnibus compilations, #9	30m 12s
omnibus compilations, #4	29m 3s
omnibus compilations, #6	29m 12s
omnibus compilations, result	3s
	5h 50m 4s

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