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.