ci: add support for aarch64-pc-windows-gnullvm target

[magic-akari]

• Closes: Support for aarch64-pc-windows-gnullvm #3297

[rami3l]

Looks like we are blocked by the upstream in a few places; I can take it from there in terms of contacting them. Thanks :)

PS: It'd be nice if rustup-init.sh can be modified to detect this new host target as well, similar to x86_64-pc-windows-gnu.

[rami3l]

@djc So the current situation in terms of the download support of this new target is:

• libcurl is not buildable, but you are dropping it anyway?
• aws-lc is not buildable, but I think I can ask the aws-lc people to look into it.

Could it be alright, say, for certain builds to use reqwest on native-tls exclusively?

[djc]

@djc So the current situation in terms of the download support of this new target is:

• libcurl is not buildable, but you are dropping it anyway?

Last I remember you didn't want to go there just yet?

• aws-lc is not buildable, but I think I can ask the aws-lc people to look into it.

That would probably be useful.

Could it be alright, say, for certain builds to use reqwest on native-tls exclusively?

Yeah, I guess we're going to need that workaround for a while given the lack of support from aws-lc-rs for a bunch of these targets.

[rami3l]

• libcurl is not buildable, but you are dropping it anyway?

Last I remember you didn't want to go there just yet?

@djc Yes, but the thing is we can totally merge this in the next release cycle. As I understand it the need for this target is useful but not essential; current ARM64 Windows users can use the MSVC build of rustup as a workaround, except the installation experience might be a bit different.

Given that, it is not necessary for us to address the libcurl error here and now.