runtime/pprof: write heap statistics to heap profile always

The heap statistics were only written if asked for a profile with debug > 0,
but that also prints a stack trace for each profile line, which is comparatively
much noisier. The statistics are short enough and separate enough
(they only appear at the end) and useful enough that we can print them
always.

This means that people using -test.memprofile in tests will get a memory
profile with statistics included now. Pprof won't care, but if people care to
look, the numbers will be there.

This avoids the need for hacks like using -memprofilerate=1 to find
the number of allocations.

Change-Id: I10a4f593403d0315aad11b37c6e554b734caa73f
Reviewed-on: https://go-review.googlesource.com/9491


Reviewed-by: David Chase <drchase@google.com>