runtime: funcfileline: get missing function name from symbol table

Copy the idea of https://golang.org/cl/92756 to funcfileline, which is
used by runtime.FuncForPC, runtime.(*Frames).Next, and others.

Change-Id: I62226f8a3464d1e55b95a1b3317a226d52a93c15
Reviewed-on: https://go-review.googlesource.com/96175
Reviewed-by: Than McIntosh <thanm@google.com>
3 files changed