runtime: remove the fake mstart caller in systemstack on linux/loong64
The backtrace knows to stop in the system stack due to writing to the SP,
so here the fake mstart caller in the system stack is no longer needed and
can be removed
ref. CL 288799
Change-Id: I0841e75fd515cf6a0d98abe4cffc3f63fc275e0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/416035
Auto-Submit: Wayne Zuo <wdvxdr@golangcn.org> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: WANG Xuerui <git@xen0n.name> Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>