Roll buildtools forward and fix GN build for it.

build/toolchain/win/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
   # Path to the directory containing the VC binaries for the right
   # combination of host and target architectures. Currently only the
   # 64-bit host toolchain is supported, with either 32-bit or 64-bit targets.
   # If vc_bin_dir is not specified on the command line (and it normally
   # isn't), we will dynamically determine the right value to use at runtime.
@@ skipping 103 matching lines @@
       command = "$python_path gyp-win-tool rc-wrapper $env rc.exe {{defines}} {{include_dirs}} /fo{{output}} {{source}}"
       outputs = [
         "{{target_out_dir}}/{{target_output_name}}/{{source_name_part}}.res",
       ]
       description = "RC {{output}}"
     }
 
     tool("asm") {
       # TODO(brettw): "/safeseh" assembler argument is hardcoded here. Extract
       # assembler flags to a variable like cflags. crbug.com/418613
-      command = "$python_path gyp-win-tool asm-wrapper $env ml.exe {{defines}} {{include_dirs}} /safeseh /c /Fo {{output}} {{source}}"
+      command = "$python_path gyp-win-tool asm-wrapper $env ml.exe {{defines}} {{include_dirs}} {{asmflags}} /safeseh /c /Fo {{output}} {{source}}"
       description = "ASM {{output}}"
       outputs = [
         "{{target_out_dir}}/{{target_output_name}}/{{source_name_part}}.obj",
       ]
     }
 
     tool("alink") {
       rspfile = "{{output}}.rsp"
       command = "$python_path gyp-win-tool link-wrapper $env False lib.exe /nologo /ignore:4221 /OUT:{{output}} @$rspfile"
       description = "LIB {{output}}"
@@ skipping 109 matching lines @@
   }
   msvc_toolchain("clang_x64") {
     environment = "environment.x64"
     current_cpu = "x64"
     prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
                          root_build_dir)
     cl = "${goma_prefix}$prefix/clang-cl.exe"
     is_clang = true
   }
 }