Add native Windows arm64 binary

[dennisameling]

I'm building https://github.com/signalapp/signal-desktop for Windows arm64, but the release script is failing due to no arm64 7za.exe binary being available for Windows:

⨯ cannot execute  cause=exec: "C:\\actions-runner\\_work\\signal-desktop-silicon\\signal-desktop-silicon\\node_modules\\7zip-bin\\win\\arm64\\7za.exe": file does not exist

This PR adds the missing binary.

I built the arm64 version as follows:

• Download the 7-Zip 21.02 Alpha source code from https://www.7-zip.org/download.html
• Open a cross-compilation command prompt as described here
• Run NMAKE PLATFORM=arm64 in the CPP\7zip folder
• Get the generated executable from CPP\7zip\Bundles\Alone\arm64\7za.exe

Can confirm this works on my arm64 device when compiling Signal Desktop.

Even though Windows arm64 can run ia32 executables through emulation, the native arm64 version is ~45% faster:

~33s on arm64 native
~48s on ia32 through emulation

 • building        target=nsis file=release\signal-desktop-win-5.3.0-beta.2.exe archs=arm64 oneClick=true perMachine=false

[dennisameling]

Hi @develar, just wanted to bring this to your attention. Would appreciate if you could have a look at this PR - happy to help/provide context or info if needed. Thanks in advance!