runtime: support symbolic backtrace of C code in a cgo crash

runtime: support symbolic backtrace of C code in a cgo crash

The new function runtime.SetCgoTraceback may be used to register stack
traceback and symbolizer functions, written in C, to do a stack
traceback from cgo code.

There is a sample implementation of runtime.SetCgoSymbolizer at
github.com/ianlancetaylor/cgosymbolizer.  Just importing that package is
sufficient to get symbolic C backtraces.

Currently only supported on linux/amd64.

Change-Id: If96ee2eb41c6c7379d407b9561b87557bfe47341
Reviewed-on: https://go-review.googlesource.com/17761
Reviewed-by: Austin Clements <austin@google.com>