runtime: update comment for Callers

Attempt to clear up confusion about how to turn
the PCs reported by Callers into the file and line
number people actually want.

Fixes issue 7690.

LGTM=r, chris.cs.guy
R=r, chris.cs.guy
CC=golang-codereviews
https://codereview.appspot.com/163550043
runtime: fix line number in first stack frame in printed stack trace

Originally traceback was only used for printing the stack
when an unexpected signal came in. In that case, the
initial PC is taken from the signal and should be used
unaltered. For the callers, the PC is the return address,
which might be on the line after the call; we subtract 1
to get to the CALL instruction.

Traceback is now used for a variety of things, and for
almost all of those the initial PC is a return address,
whether from getcallerpc, or gp->sched.pc, or gp->syscallpc.
In those cases, we need to subtract 1 from this initial PC,
but the traceback code had a hard rule "never subtract 1
from the initial PC", left over from the signal handling days.

Change gentraceback to take a flag that specifies whether
we are tracing a trap.

Change traceback to default to "starting with a return PC",
which is the overwhelmingly common case.

Add tracebacktrap, like traceback but starting with a trap PC.

Use tracebacktrap in signal handlers.

Fixes issue 7690.

LGTM=iant, r
R=r, iant
CC=golang-codereviews
https://codereview.appspot.com/167810044