internal/abi, internal/buildcfg: always enable register ABI on loong64

internal/abi, internal/buildcfg: always enable register ABI on loong64

Change-Id: Ia3a31556c8d355eb6bdb42d3b3c8bc6b37311ed9
Reviewed-on: https://go-review.googlesource.com/c/go/+/525575
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>