dtrace/powerpc: "Fix" stack traces across trap frames

dtrace/powerpc: "Fix" stack traces across trap frames

In function boundary tracing the link register is not yet saved to the
save stack location, so the save point contains whatever the previous
'lr' save was, or even garbage, at the time the trap is taken.  Address
this by explicitly loading the link register from the trap frame instead
of the stack, and propagate that out.