gn win: Template toolchain to support x86 and x64
gn win: Template toolchain to support x86 and x64

Bug: crashpad:79
Change-Id: Ib94d54c226e1999f45b56e8a73c8a3c057b75dc3
Reviewed-on: https://chromium-review.googlesource.com/905622
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/mini_chromium,chromium/mini_chromium,chromium/mini_chromium