Instead of a per-goroutine stack of defers for all sizes,
introduce per-P defer pool for argument sizes 8, 24, 40, 56, 72 bytes.

For a program that starts 1e6 goroutines and then joins then:
old: rss=6.6g virtmem=10.2g time=4.85s
new: rss=4.5g virtmem= 8.2g time=3.48s

R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/42750044