runtime: align end of systemstack_switch prologue on amd64 for consistency with gosave_systemstack_switch
gosave_systemstack_switch saves PC with fixed offset of 8 bytes from systemstack_switch to bypass prologue.
This commit makes this offset consistent with actual address of UNDEF instruction intended to be at that address.
Fixes #71440
Change-Id: Ibe6458c5bcb0bdaec228a2f13d6aec7ecc0e319e
Reviewed-on: https://go-review.googlesource.com/c/go/+/740360
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>