[BrowserSwitcher] Build BHO for both bitnesses

This is a reland of this change, with a fix for symbol_level=0:
https://chromium-review.googlesource.com/c/chromium/src/+/1589457

To do so, this CL also adds support for the x86 toolchain in an x64
build.

Instead of generating one DLL, we now generate 2 DLLs in the root
out-dir:

- browser_switcher_bho.dll (x86)
- browser_switcher_bho_64.dll (x64)

The one that is cross-built (i.e. bitness doesn't match the host) is
built in a sub-directory, and then copied to the root out-dir.

Bug: 952897
Change-Id: Ie18544fb97ba3091bc8113932e634c539d6d7502
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1653660
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668482}

• BUILD.gn[diff]
• build/toolchain/win/BUILD.gn[diff]
• build/toolchain/win/midl.py[diff]
• chrome/browser/browser_switcher/bho/BUILD.gn[diff]