jmpdefer modifies PC, SP, and LR, and not atomically,
so walking past jmpdefer will often end up in a state
where the three are not a consistent execution snapshot.
This was causing warning messages a few frames later
when the traceback realized it was confused, but given
the right memory it could easily crash instead.

Update #8153

LGTM=minux, iant
R=golang-codereviews, minux, iant
CC=golang-codereviews, r
https://golang.org/cl/107970043