Add first support for Linux arm64

[dennisameling]

Currently, trying to install Git Credential Manager Core on Linux arm64 (e.g. WSL2 on the Surface Pro X or Raspberry Pi 4) results in the following error:

dennis@DESKTOP-8HTP3NV:~/repos$ sudo dpkg -i gmcore.deb
dpkg: error processing archive gmcore.deb (--install):
 package architecture (amd64) does not match system (arm64)
Errors were encountered while processing:
 gmcore.deb

This PR adds support for Linux arm64 builds, both when building natively on arm64 or while cross-compiling from an x86 host (in which case you can now run dotnet build -c LinuxDebug -r linux-arm64).

Can confirm it works on my arm64 Surface Pro X through WSL2 Ubuntu 🎉:

dennis@DESKTOP-8HTP3NV:~/repos/gmcoretest$ sudo dpkg -i gcmcore-linux_arm64.2.0.301.22928.deb
[sudo] password for dennis:
Selecting previously unselected package gcmcore.
(Reading database ... 40440 files and directories currently installed.)
Preparing to unpack gcmcore-linux_arm64.2.0.301.22928.deb ...
Unpacking gcmcore (2.0.301.22928) ...
Setting up gcmcore (2.0.301.22928) ...

dennis@DESKTOP-8HTP3NV:~/repos/gmcoretest$ git-credential-manager-core --version
Git Credential Manager version 2.0.301-beta+90596ecdf4 (Linux, .NET Core 3.1.10)

Things that still need to happen AFAIK:

• Add support for signing multiple .deb files at once in .github/run_esrp_signing.py, it currently only looks at the first .deb file it finds. How does that sound?
• Running tests on arm64. This is possible e.g. with Travis (https://docs.travis-ci.com/user/multi-cpu-architectures/#multi-cpu-availaibility); please let me know if only running tests on amd64 is enough for now or whether you'd like me to set up a Travis config for running tests on arm64 as well.

[ghost]

All CLA requirements met.

[hickford]

If it helps anyone else, here's how I built for 32-bit linux-arm:

#466 (comment)

[theofficialgman]

Any chance on having this updated for github actions and merged?