cmd/go: add "syscall" to the set of packages that run extFiles++

This eliminates the need for syscall/asm.s, which is now empty.

Change-Id: Ied060195e03e9653251f54ea8ef6572444b37fdf
Reviewed-on: https://go-review.googlesource.com/36844Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/work/build.go

@@ -2189,7 +2189,7 @@ func (gcToolchain) gc(b *Builder, p *load.Package, archive, obj string, asmhdr b
 	extFiles := len(p.CgoFiles) + len(p.CFiles) + len(p.CXXFiles) + len(p.MFiles) + len(p.FFiles) + len(p.SFiles) + len(p.SysoFiles) + len(p.SwigFiles) + len(p.SwigCXXFiles)
 	if p.Standard {
 		switch p.ImportPath {
-		case "bytes", "internal/poll", "net", "os", "runtime/pprof", "sync", "time":
+		case "bytes", "internal/poll", "net", "os", "runtime/pprof", "sync", "syscall", "time":
 			extFiles++
 		}
 	}

src/syscall/asm.s

-// Copyright 2014 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.
-
-// Dummy asm file to keep Windows building.
-// TODO(sameer): delete this after updating go tool
-// per Ian's comment in golang.org/cl/36716