cpu: replace specific instructions with WORD in the function get_cpucfg on loong64

The CPUCFG instruction on loong64 was introduced in Go1.24, which caused
compilation errors when using this feature in Go1.23 and earlier versions.

Change-Id: I68891bbfc527194ecdafebac3398e700bfb53c2f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/656915
Reviewed-by: Meidan Li <limeidan@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>