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>

doc/go1.15.html

@@ -246,7 +246,11 @@ TODO
 </p>
 
 <p><!-- CL 222661 -->
-  TODO: -spectre flags
+  Go 1.15 adds a <code>-spectre</code> flag to both the
+  compiler and the assembler, to allow enabling Spectre mitigations.
+  These should almost never be needed and are provided mainly as a
+  “defense in depth” mechanism.
+  See the <a href="https://github.com/golang/go/wiki/Spectre">Spectre wiki page</a> for details.
 </p>
 
 <h2 id="linker">Linker</h2>
@@ -276,7 +280,7 @@ TODO
   These changes are part of a multi-release project
   to <a href="https://golang.org/s/better-linker">modernize the Go
   linker</a>, meaning that there will be additional linker
-  improvements expected in future releases. 
+  improvements expected in future releases.
 </p>
 
 <h2 id="library">Core library</h2>

src/cmd/asm/doc.go

@@ -33,14 +33,17 @@ Flags:
 		Dump instructions as they are parsed.
 	-dynlink
 		Support references to Go symbols defined in other shared libraries.
+	-gensymabis
+		Write symbol ABI information to output file. Don't assemble.
 	-o file
 		Write output to file. The default is foo.o for /a/b/c/foo.s.
 	-shared
 		Generate code that can be linked into a shared library.
+	-spectre list
+		Enable spectre mitigations in list (all, ret).
 	-trimpath prefix
 		Remove prefix from recorded source file paths.
-	-gensymabis
-		Write symbol ABI information to output file. Don't assemble.
+
 Input language:
 
 The assembler uses mostly the same syntax for all architectures,

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