Context info could be obtained from other sources (see below) no only from
td_pcb field
- Do not show a0..a3 values unless they're obtained from the stack. These
are only confirmed values.
- Fix bt command in DDB. Previous implementation used thread's trapframe
structure as a source info for trace unwinding, but this structure
is filled only when exception occurs. Valid register values for sleeping
processes are in pcb_context array. For curthread use pc/sp/ra for current
frame
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