runtime: add Frame.Entry field

This enables compatibility with golang.org/x/tools/internal/pkgbits.

https://github.com/golang/tools/blob/master/internal/pkgbits/frames_go17.go

src/runtime/symtab.go

@@ -13,6 +13,8 @@ type Frame struct {
 
 	File string
 	Line int
+
+	Entry uintptr
 }
 
 func CallersFrames(callers []uintptr) *Frames {