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Cache 2KB, 4KB, 8KB, and 16KB stacks.  Larger stacks
will be allocated directly.  There is no point in cacheing
32KB+ stacks as we ask for and return 32KB at a time
from the allocator.

Note that the minimum stack is 8K on windows/64bit and 4K on
windows/32bit and plan9.  For these os/arch combinations,
the number of stack orders is less so that we have the same
maximum cached size.

Fixes #9045

Change-Id: Ia4195dd1858fb79fc0e6a91ae29c374d28839e44
Reviewed-on: https://go-review.googlesource.com/2098

Reviewed-by: Russ Cox <rsc@golang.org>