runtime: move cgo traceback into unwinder
Currently, gentraceback's loop ends with a call to tracebackCgoContext
to process cgo frames. This requires spreading various parts of the
printing and pcbuf logic across these two functions.
Clean this up by moving cgo unwinding into unwinder and then lifting
the printing and pcbuf logic from tracebackCgoContext into
gentraceback along with the other printing and pcbuf logic.
Updates #54466.
Change-Id: Ic71afaa5ae110c0ea5be9409e267e4284e36a8c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/468299
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>