Currently MaxGomaxprocs is 256. The previous CL saved enough per-P
static space that we can quadruple MaxGomaxprocs (and hence the static
size of allp) and still come out ahead.

This is safe for Go 1.9. In Go 1.10 we'll eliminate the hard-coded
limit entirely.

Updates #15131.

Change-Id: I919ea821c1ce64c27812541dccd7cd7db4122d16
Reviewed-on: https://go-review.googlesource.com/45673
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>