runtime: make Malloc benchmarks actually benchmark malloc

runtime: make Malloc benchmarks actually benchmark malloc

The compiler is too clever so the allocations are currently
avoided. Rewrite to make them actually allocate.

Change-Id: I9542e1365120b2ace318360883b0b01ed5670da7
Reviewed-on: https://go-review.googlesource.com/c/go/+/449476
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>