Fix negative mutator time in GC stats in prof builds
Because garbage collector calls `retainerProfile()` and `heapCensus()`, GC times normally include some of PROF times too. To fix this we have these lines: // heapCensus() is called by the GC, so RP and HC time are // included in the GC stats. We therefore subtract them to // obtain the actual GC cpu time. stats.gc_cpu_ns -= prof_cpu; stats.gc_elapsed_ns -= prof_elapsed; These variables are later used for calculating GC time excluding the final GC (which should be attributed to EXIT). exit_gc_elapsed = stats.gc_elapsed_ns - start_exit_gc_elapsed; The problem is if we subtract PROF times from `gc_elapsed_ns` and then subtract `start_exit_gc_elapsed` from the result, we end up subtracting PROF times twice, because `start_exit_gc_elapsed` also includes PROF times. We now subtract PROF times from GC after the calculations for EXIT and MUT times. The existing assertion that checks INIT + MUT + GC + EXIT = TOTAL now holds. When we subtract PROF numbers from GC, and a new assertion INIT + MUT + GC + PROF + EXIT = TOTAL also holds. Fixes #15897. New assertions added in this commit also revealed #16102, which is also fixed by this commit.