reflect: implement StructOf

This change exposes a facility to create new struct types from a slice of
reflect.StructFields.

- reflect: implement StructOf
- reflect: tests for StructOf
- runtime: generate typelinks for structs

Fixes golang#5748.

Change-Id: I3b8fd4fadd6ce3b1b922e284f0ae72a3a8e3ce44

Author: sbinet

src/cmd/internal/gc/reflect.go

@@ -1226,7 +1226,7 @@ ok:
 				break
 			}
 			fallthrough
-		case TARRAY, TCHAN, TFUNC, TMAP:
+		case TARRAY, TCHAN, TFUNC, TMAP, TSTRUCT:
 			slink := typelinksym(t)
 			dsymptr(slink, 0, s, 0)
 			ggloblsym(slink, int32(Widthptr), int8(dupok|obj.RODATA))