cmd/asm, cmd/compile, runtime: add -spectre=ret mode

This commit extends the -spectre flag to cmd/asm and adds
a new Spectre mitigation mode "ret", which enables the use
of retpolines.

Retpolines prevent speculation about the target of an indirect
jump or call and are described in more detail here:
https://support.google.com/faqs/answer/7625886

Change-Id: I4f2cb982fa94e44d91e49bd98974fd125619c93a
Reviewed-on: https://go-review.googlesource.com/c/go/+/222661
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/internal/obj/link.go

@@ -653,6 +653,7 @@ type Link struct {
 	Flag_optimize      bool
 	Flag_locationlists bool
 	Flag_newobj        bool // use new object file format
+	Retpoline          bool // emit use of retpoline stubs for indirect jmp/call
 	Bso                *bufio.Writer
 	Pathname           string
 	hashmu             sync.Mutex       // protects hash, funchash