cmd/compile,internal/runtime/maps: stack allocated maps and small alloc

cmd/compile,internal/runtime/maps: stack allocated maps and small alloc

The compiler will stack allocate the Map struct and initial group if
possible.

Stack maps are initialized inline without calling into the runtime.
Small heap allocated maps use makemap_small.

These are the same heuristics as existing maps.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I6c371d1309716fd1c38a3212d417b3c76db5c9b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/622042
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>