ARM64EC is a new build target for Windows ARM64 devices for x64 support.
Currently that build flavor fails due to attempting to use x64 intrinsics.
This commit fixes it by changing the auto-detection to be aarch64
instead of x64 for arm64ec.
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24119>