cmd/cgo: allow for stdcall decorated dynimport names

To allow for stdcall decorated names on Windows, two changes were needed:
1. Change the symbol versioning delimiter '@' in cgo's dynimport output to a '#', and in cmd/ld when it parses dynimports.
2. Remove the "@N" decorator from the first argument of cgo's dynimport output (PE only).

Fixes #4607.

R=minux.ma, adg, rsc
CC=golang-dev
https://golang.org/cl/7047043

src/cmd/cgo/out.go

@@ -176,7 +176,7 @@ func dynimport(obj string) {
 		for _, s := range sym {
 			targ := s.Name
 			if s.Version != "" {
-				targ += "@" + s.Version
+				targ += "#" + s.Version
 			}
 			fmt.Fprintf(stdout, "#pragma dynimport %s %s %q\n", s.Name, targ, s.Library)
 		}
@@ -218,7 +218,8 @@ func dynimport(obj string) {
 		}
 		for _, s := range sym {
 			ss := strings.Split(s, ":")
-			fmt.Fprintf(stdout, "#pragma dynimport %s %s %q\n", ss[0], ss[0], strings.ToLower(ss[1]))
+			name := strings.Split(ss[0], "@")[0]
+			fmt.Fprintf(stdout, "#pragma dynimport %s %s %q\n", name, ss[0], strings.ToLower(ss[1]))
 		}
 		return
 	}

src/cmd/ld/go.c

@@ -506,7 +506,7 @@ loaddynimport(char *file, char *pkg, char *p, int n)
 		}
 
 		name = expandpkg(name, pkg);
-		q = strchr(def, '@');
+		q = strchr(def, '#');
 		if(q)
 			*q++ = '\0';
 		s = lookup(name, 0);