runtime: copy sqrt normalization bugfix from math

This copies the change from CL 16158 (applied as
22d4c8bf).

Updates #13013

Change-Id: Id7d02e63d92806f06a4e064a91b2fb6574fe385f
Reviewed-on: https://go-review.googlesource.com/16291Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sqrt.go

@@ -117,7 +117,7 @@ func sqrt(ix uint64) uint64 {
 	// normalize x
 	exp := int((ix >> float64Shift) & float64Mask)
 	if exp == 0 { // subnormal x
-		for ix&1<<float64Shift == 0 {
+		for ix&(1<<float64Shift) == 0 {
 			ix <<= 1
 			exp--
 		}

src/runtime/sqrt_test.go

@@ -74,6 +74,7 @@ var vfsqrtSC = []float64{
 	0,
 	math.Inf(1),
 	math.NaN(),
+	math.Float64frombits(2),
 }
 var sqrtSC = []float64{
 	math.NaN(),
@@ -82,4 +83,5 @@ var sqrtSC = []float64{
 	0,
 	math.Inf(1),
 	math.NaN(),
+	3.1434555694052576e-162,
 }