In runtime.gopanic, the _panic object p is stack allocated and
referenced from gp._panic. With stack objects, p on stack is dead
at the point preprintpanics runs. gp._panic points to p, but
stack scan doesn't look at gp. Heap scan of gp does look at
gp._panic, but it stops and ignores the pointer as it points to
the stack. So whatever p points to may be collected and clobbered.
We need to scan gp._panic explicitly during stack scan.
To test it reliably, we introduce a GODEBUG mode "clobberfree",
which clobbers the memory content when the GC frees an object.