cmd/compile: accept old and new import format for builtin declarations

Test with forceNewExport set to true (but continues to be disabled by
default for now).

Fixes #15322.

Change-Id: I3b893db2206cbb79e66339284f22f4a0b20bf137
Reviewed-on: https://go-review.googlesource.com/22328Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/main.go

@@ -643,11 +643,24 @@ func loadsys() {
 	iota_ = -1000000
 	incannedimport = 1
 
-	importpkg = Runtimepkg
-	parse_import(bufio.NewReader(strings.NewReader(runtimeimport)), nil)
-
-	importpkg = unsafepkg
-	parse_import(bufio.NewReader(strings.NewReader(unsafeimport)), nil)
+	// The first byte in the binary export format is a 'c' or 'd'
+	// specifying the encoding format. We could just check that
+	// byte, but this is a perhaps more robust. Also, it is not
+	// speed-critical.
+	// TODO(gri) simplify once textual export format has gone
+	if strings.HasPrefix(runtimeimport, "package") {
+		// textual export format
+		importpkg = Runtimepkg
+		parse_import(bufio.NewReader(strings.NewReader(runtimeimport)), nil)
+		importpkg = unsafepkg
+		parse_import(bufio.NewReader(strings.NewReader(unsafeimport)), nil)
+	} else {
+		// binary export format
+		importpkg = Runtimepkg
+		Import(bufio.NewReader(strings.NewReader(runtimeimport)))
+		importpkg = unsafepkg
+		Import(bufio.NewReader(strings.NewReader(unsafeimport)))
+	}
 
 	importpkg = nil
 	incannedimport = 0