[mono] Use $(HostArch) for LLVM packages

[EgorBo]

Currently we always download x64 packages -- it's ok for our workloads since we can use them for cross-compilation to arm64 but it's not possbile to use them when you compile dotnet/runtime on an arm64 machine.

Fixes #34311

@akoeplinger should I also duplicate these: https://github.com/dotnet/runtime/blob/master/eng/Versions.props#L127-L132 ?

[am11]

<runtimelinuxx64MicrosoftNETCoreRuntimeMonoLLVMSdkVersion>

would be nice if we make the property names platform agnostic, as we can alway use Condition attribute to target a platform when needed: i.e. <runtimeMicrosoftNETCoreRuntimeMonoLLVMSdkVersion>.

[akoeplinger]

You don't need to duplicate the Versions.props, we're using the same version anyway. There's an issue somewhere about turning the LLVM package into a proper meta-package so we don't need these explicit package references anymore.

[filipnavara]

Thanks a lot!

[am11]

Does it matter that ToLowerInvariant is missing parenthesis?

[EgorBo]

Oops

[EgorBo]

@am11 however it works without them too

[am11]

Great, i was also thinking that it (might) just work in property functions. 👍