cmd/link: include DW_AT_producer in .debug_info

This can make life easier for Delve (and other debuggers),
and can help them with bug reports.

Sample producer field (from objdump):
<48> DW_AT_producer : Go cmd/compile devel +8a59dbf41a Mon May 8 16:02:44 2017 -0400

Change-Id: I0605843c959b53a60a25a3b870aa8755bf5d5b13
Reviewed-on: https://go-review.googlesource.com/33588
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/dwarf/dwarf.go

@@ -197,6 +197,7 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 			{DW_AT_high_pc, DW_FORM_addr},
 			{DW_AT_stmt_list, DW_FORM_data4},
 			{DW_AT_comp_dir, DW_FORM_string},
+			{DW_AT_producer, DW_FORM_string},
 		},
 	},
 

src/cmd/link/internal/ld/dwarf.go

@@ -1024,6 +1024,8 @@ func writelines(ctxt *Link, syms []*Symbol) ([]*Symbol, []*Symbol) {
 	// OS X linker requires compilation dir or absolute path in comp unit name to output debug info.
 	compDir := getCompilationDir()
 	newattr(dwinfo, dwarf.DW_AT_comp_dir, dwarf.DW_CLS_STRING, int64(len(compDir)), compDir)
+	producer := "Go cmd/compile " + objabi.Version
+	newattr(dwinfo, dwarf.DW_AT_producer, dwarf.DW_CLS_STRING, int64(len(producer)), producer)
 
 	// Write .debug_line Line Number Program Header (sec 6.2.4)
 	// Fields marked with (*) must be changed for 64-bit dwarf