runtime: add benchmark of heapBitsSetType

There was an old benchmark that measured this indirectly
via allocation, but I don't understand how to factor out the
allocation cost when interpreting the numbers.

Replace with a benchmark that only calls heapBitsSetType,
that does not allocate. This was not possible when the
benchmark was first written, because heapBitsSetType had
not been factored out of mallocgc.

Change-Id: I30f0f02362efab3465a50769398be859832e6640
Reviewed-on: https://go-review.googlesource.com/9701


Reviewed-by: Austin Clements <austin@google.com>