sysUsed on Windows cares about the result from the VirtualAlloc syscall
returning exactly the address that was passed to it. However,
VirtualAlloc aligns the address its given to the kernel's allocation
granularity, so the returned address may not be the same.

Note that this wasn't an issue in the past because we only sysUsed
regions owned by spans, and spans are always a multiple of 8K, which
is a multiple of the allocation granularity on most Windows machines.

Change-Id: I3f5ccd63c6bbbd8b7995945ecedee17573b31667
Reviewed-on: https://go-review.googlesource.com/c/153677
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>