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>

test/codegen/retpoline.go

@@ -0,0 +1,14 @@
+// +build amd64
+// asmcheck -gcflags=-spectre=ret
+
+package codegen
+
+func CallFunc(f func()) {
+	// amd64:`CALL\truntime.retpoline`
+	f()
+}
+
+func CallInterface(x interface{ M() }) {
+	// amd64:`CALL\truntime.retpoline`
+	x.M()
+}