runtime: compute size classes statically

No point in computing this info on startup.
Compute it at build time.
This lets us spend more time computing & checking the size classes.

Improve the div magic for rounding to the start of an object.
We can now use 32-bit multiplies & shifts, which should help
32-bit platforms.

The static data is <1KB.

The actual size classes are not changed by this CL.

Change-Id: I6450cec7d1b2b4ad31fd3f945f504ed2ec6570e7
Reviewed-on: https://go-review.googlesource.com/32219


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>