runtime: move loong64 library entry point to os-agnostic file

runtime: move loong64 library entry point to os-agnostic file

The library entry point for loong64 is agnostic to the OS, so move it to
asm_loong64.s. This is similar to what we do for other architectures.

Cq-Include-Trybots: luci.golang.try:gotip-linux-loong64
Change-Id: I6915eb76d3ea72a779e05e78d85f24793169c61f
Reviewed-on: https://go-review.googlesource.com/c/go/+/706416
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>