The move from 4kB to 8kB in Go 1.2 was to eliminate many stack split hot spots.

The move back to 4kB was predicated on copying stacks eliminating
the potential for hot spots.

Unfortunately, the fact that stacks do not copy 100% of the time means
that hot spots can still happen under the right conditions, and the slowdown
is worse now than it was in Go 1.2. There is a real program in issue 8030 that
sees about a 30x slowdown: it has a reflect call near the top of the stack
which inhibits any stack copying on that segment.

Go back to 8kB until stack copying can be used 100% of the time.

Fixes #8030.

LGTM=khr, dave, iant
R=iant, khr, r, bradfitz, dave
CC=golang-codereviews
https://golang.org/cl/92540043