runtime: don't restore from g0.sched in systemstack on arm64
On arm64, systemstack restores the frame pointer from g0.sched to R29
prior to calling the callback. That doesn't really make any sense. The
frame pointer value in g0.sched is some arbitrary BP from a prior
context save, but that is not the caller of systemstack.
amd64 does not do this. In fact, it leaves BP completely unmodified so
frame pointer unwinders like gdb can walk through the systemstack frame
and continue traceback on the caller's stack. Unlike mcall, systemstack
always returns to the original goroutine, so that is safe.
We should do the same on arm64.
For #63630.
Cq-Include-Trybots: luci.golang.try:gotip-linux-arm64-longtest
Change-Id: I6a6a636c35d321dd5d7dc1c4d09e29b55b1ab621
Reviewed-on: https://go-review.googlesource.com/c/go/+/669236 Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com> Reviewed-by: Nick Ripley <nick.ripley@datadoghq.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>