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This patch is the first step towards shipping dxcompiler.dll and dxil.dll (also referred to as DXC) on Windows for WebGPU. With this patch DXC is disabled by default at build time and can be enabled on x86_64 by adding `ac_add_options --enable-dxcompiler` in the mozconfig file. Followup work will include in no particular order: - Enabling DXC by default. - Enabling DXC for Windows on x86 and arm architectures. - Using our own DXC builds instead of pulling the dlls from the Windows SDK. Differential Revision: https://phabricator.services.mozilla.com/D216293