runtime: switch to systemstack before throw in casgstatus
CL 580255 increased the frame size of entersyscall and reentersyscall,
which is causing the x/sys repository to fail to build for
windows/arm64 because of an overflow of the nosplit stack reservation.
Fix this by wrapping the other call to throw in casgstatus in a system
stack switch. This is a fatal throw anyway indicating a core runtime
invariant is broken, so this path is basically never taken. This cuts
off the nosplit frame chain and allows x/sys to build.
Change-Id: I00b16c9db3a7467413ed48953c7f8a9a750f000a
Reviewed-on: https://go-review.googlesource.com/c/go/+/580775 Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>