cmd/asm, cmd/compile, doc: document -spectre flags

Most of the docs are in the new wiki page
https://golang.org/wiki/Spectre.

Updates #37419.

Change-Id: I6e8f76670593c089de895e1665b41d874f879df9
Reviewed-on: https://go-review.googlesource.com/c/go/+/236599
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/compile/doc.go

@@ -107,6 +107,8 @@ Flags:
 		Warn about composite literals that can be simplified.
 	-shared
 		Generate code that can be linked into a shared library.
+	-spectre list
+		Enable spectre mitigations in list (all, index, ret).
 	-traceprofile file
 		Write an execution trace to file.
 	-trimpath prefix