runtime: "traceback: unexpected SPWRITE function runtime.systemstack"

[prattmic]

https://go.dev/cl/525835 (backported to 1.21.2) introduced a regression causing a throw when panicking on the system stack:

traceback: unexpected SPWRITE function runtime.systemstack
runtime.systemstack(0x501fcc0)
    /Users/cherryyz/src/go/src/runtime/asm_amd64.s:509 +0x4a fp=0x7ff7bbfac760 sp=0x7ff7bbfac750 pc=0x4fa738a

This was fixed in https://go.dev/cl/531815 at tip, but needs a backport to 1.21.

[prattmic]

@gopherbot please backport to 1.21. This fixes a regression in a previous backport (#62464).

[gopherbot]

Backport issue(s) opened: #64935 (for 1.21).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.