cmd/compile: specialize map creation for small hint sizes

Handle make(map[any]any) and make(map[any]any, hint) where
hint <= BUCKETSIZE special to allow for faster map initialization
and to improve binary size by using runtime calls with fewer arguments.

Given hint is smaller or equal to BUCKETSIZE in which case
overLoadFactor(hint, 0)  is false and no buckets would be allocated by makemap:
* If hmap needs to be allocated on the stack then only hmap's hash0
  field needs to be initialized and no call to makemap is needed.
* If hmap needs to be allocated on the heap then a new special
  makehmap function will allocate hmap and intialize hmap's
  hash0 field.

Reduces size of the godoc by ~36kb.

AMD64
name         old time/op    new time/op    delta
NewEmptyMap    16.6ns ± 2%     5.5ns ± 2%  -66.72%  (p=0.000 n=10+10)
NewSmallMap    64.8ns ± 1%    56.5ns ± 1%  -12.75%  (p=0.000 n=9+10)

Updates #6853

Change-Id: I624e90da6775afaa061178e95db8aca674f44e9b
Reviewed-on: https://go-review.googlesource.com/61190
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/export_test.go

@@ -377,11 +377,16 @@ func (rw *RWMutex) Unlock() {
 	rw.rw.unlock()
 }
 
-func MapBuckets(m map[int]int) int {
+func MapBucketsCount(m map[int]int) int {
 	h := *(**hmap)(unsafe.Pointer(&m))
 	return 1 << h.B
 }
 
+func MapBucketsPointerIsNil(m map[int]int) bool {
+	h := *(**hmap)(unsafe.Pointer(&m))
+	return h.buckets == nil
+}
+
 func LockOSCounts() (external, internal uint32) {
 	g := getg()
 	if g.m.lockedExt+g.m.lockedInt == 0 {