runtime: implement dumpmemstats in terms of readmemstats_m
Since MemStats is now populated directly and some values are derived,
avoid duplicating the logic by instead populating the heap dump directly
from MemStats (external version) instead of memstats (runtime internal
version).
Change-Id: I0bec96bfa02d2ffd1b56475779c124a760e64238
Reviewed-on: https://go-review.googlesource.com/c/go/+/255817
Trust: Michael Knyszek <mknyszek@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>