math, cmd/internal/gc, cmd/7g: enable SQRT inlining, add assembly implementation

benchmark                 old ns/op     new ns/op     delta
BenchmarkSqrt             474           16.5          -96.52%
BenchmarkSqrtIndirect     476           38.1          -92.00%
BenchmarkSqrtGo           484           483           -0.21%

Change-Id: I5ad0132feda0d3275a884523b9e79d83db4fc726
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8465
Reviewed-by: David Crawshaw <crawshaw@golang.org>
diff --git a/src/cmd/internal/gc/walk.go b/src/cmd/internal/gc/walk.go
index a0a29d3..c845f78 100644
--- a/src/cmd/internal/gc/walk.go
+++ b/src/cmd/internal/gc/walk.go
@@ -624,7 +624,7 @@
 
 		if n.Left.Op == ONAME && n.Left.Sym.Name == "Sqrt" && n.Left.Sym.Pkg.Path == "math" {
 			switch Thearch.Thechar {
-			case '5', '6':
+			case '5', '6', '7':
 				n.Op = OSQRT
 				n.Left = n.List.N
 				n.List = nil