runtime: align taggable pointers more so we can use low bits for tag

Currently we assume alignment to 8 bytes, so we can steal the low 3 bits.
This CL assumes alignment to 512 bytes, so we can steal the low 9 bits.

That's 6 extra bits!

Aligning to 512 bytes wastes a bit of space but it is not egregious.
Most of the objects that we make tagged pointers to are pretty big.

Update #49405

Change-Id: I66fc7784ac1be5f12f285de1d7851d5a6871fb75
Reviewed-on: https://go-review.googlesource.com/c/go/+/665815


Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>