internal/buildcfg: enable greenteagc experiment for loong64

internal/buildcfg: enable greenteagc experiment for loong64

The loong64 compiler bug has been resolved, so we can now
unconditionally enable the experiment on the architecture.

Updates #73581
Fixes #75776

Change-Id: I390f8a125d43ca64798ea5b6a408aaf7220fadbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/710476
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>