cmd/gc: allocate select descriptor on stack

cmd/gc: allocate select descriptor on stack

benchmark                      old ns/op     new ns/op     delta
BenchmarkSelectUncontended     220           165           -25.00%
BenchmarkSelectContended       209           161           -22.97%
BenchmarkSelectProdCons        1042          904           -13.24%

But more importantly this change will allow
to get rid of free function in runtime.

Fixes #6494.

LGTM=rsc, khr
R=golang-codereviews, rsc, dominik.honnef, khr
CC=golang-codereviews, remyoudompheng
https://golang.org/cl/107670043