pkg/cover: fix objdump process hang
pkg/cover: fix objdump process hang

One instance we observed that objdump hanged due to stdout
pipe overflow due to panic in archCallInsn.
The reason for the original panic is still unclear,
but fix the objdump hang. We need to terminate objdump
and propagate the panic.
Also extend the panic messages.

xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller