Set Distro Env HOSTTYPE to aarch64 for ARM64 build

[wangxin12]

Summary of the Pull Request

Set Distro Env HOSTTYPE to aarch64 for ARM64 build

PR Checklist

• Closes: Link to issue HOSTTYPE reports incorrect value on aarch64 systems #40040
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[benhillis]

Instead of adding more #defines, can you do something similar to what we do for WSL_PACKAGE_VERSION?

[Copilot]

Pull request overview

Updates WSL’s default distro environment so HOSTTYPE reflects the target architecture, fixing incorrect HOSTTYPE=x86_64 on ARM64 builds (issue #40040).

Changes:

• Introduce a build-time DISTRO_HOSTTYPE define set to x86_64 on x64 and aarch64 on arm64.
• Use DISTRO_HOSTTYPE when constructing the default distro environment (both the IDL macro and the service-side default environment vector).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
CMakeLists.txt	Defines DISTRO_HOSTTYPE based on TARGET_PLATFORM and exports it as a compile definition.
src/windows/service/inc/wslservice.idl	Switches LXSS_DISTRO_DEFAULT_ENVIRONMENT to build HOSTTYPE from DISTRO_HOSTTYPE.
src/windows/service/exe/DistributionRegistration.h	Updates the DefaultEnvironment default value to use DISTRO_HOSTTYPE instead of a hardcoded x86_64.