Error in accounting for GC cpu time in sequential collections
## Summary
stat_endGC accounts for cpu time consumed by a garbage collection the following fragment:
```
for (unsigned int i=0; i < par_n_threads; i++) {
gc_thread *gct = gc_threads[i];
ASSERT(gct->gc_end_cpu >= gct->gc_start_cpu);
stats.gc.cpu_ns += gct->gc_end_cpu - gct->gc_start_cpu;
}
```
however, this is incorrect when accounting for a sequential collection when n_capabilities > 1. In this case, par_n_threads will be set to 1. However the cpu stats will not necessarily be on capability 0(i.e. in gc_threads[0]) which the fragment above assumes. The stats will be on the capability of the gc leader.
EDIT:
I've also seen
```
for (i=0; i < n_gc_threads; i++) {
copied += RELAXED_LOAD(&gc_threads[i]->copied);
}
```
in GC.c seems to have the same problem. Holding off fixing this for now, since I now suspect I may be confused
## Steps to reproduce
I've not constructed a demonstration.
## Expected behavior
n/a
## Environment
* GHC version used: head
issue