cmd/asm: add doc.go

There was no documentation produced by "go doc cmd/asm".
Follow the style set by cmd/compile.

Fixes #13148.

Change-Id: I02e08ce2e7471f855bfafbbecee98ffdb7096995
Reviewed-on: https://go-review.googlesource.com/16997Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/asm/doc.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Asm, typically invoked as ``go tool asm'', assembles the source file into an object
+file named for the basename of the argument source file with a .o suffix. The
+object file can then be combined with other objects into a package archive.
+
+Command Line
+
+Usage:
+
+	go tool asm [flags] file
+
+The specified file must be a Go assembly file.
+The same assembler is used for all target operating systems and architectures.
+The GOOS and GOARCH environment variables set the desired target.
+
+Flags:
+
+	-D value
+		predefined symbol with optional simple value -D=identifer=value;
+		can be set multiple times
+	-I value
+		include directory; can be set multiple times
+	-S	print assembly and machine code
+	-debug
+		dump instructions as they are parsed
+	-dynlink
+		support references to Go symbols defined in other shared libraries
+	-o string
+		output file; default foo.o for /a/b/c/foo.s
+	-shared
+		generate code that can be linked into a shared library
+	-trimpath string
+		remove prefix from recorded source file paths
+
+Input language:
+
+The assembler uses mostly the same syntax for all architectures,
+the main variation having to do with addressing modes. Input is
+run through a simplified C preprocessor that implements #include,
+#define, #ifdef/endif, but not #if or ##.
+
+For more information, see https://golang.org/doc/asm.
+*/
+package main

src/cmd/asm/internal/flags/flags.go

@@ -36,6 +36,9 @@ func init() {
 type MultiFlag []string
 
 func (m *MultiFlag) String() string {
+	if len(*m) == 0 {
+		return ""
+	}
 	return fmt.Sprint(*m)
 }