Godefs was a C program that ran gcc and then parsed the
stabs debugging information in the resulting object file to
generate C or Go code for bootstrapping as part of
package runtime or package syscall.

Cgo does the same work, but using the dwarf debugging
information.  Add -godefs and -cdefs options to cgo that
mimic godefs's output, albeit with different input
(a Go program, not a C program).

This has been a "nice to have" for a while but was forced
by Apple removing stabs debugging output from their
latest compilers.

Fixes #835.
Fixes #2338.

R=golang-dev, bradfitz, r, dave, iant
CC=golang-dev
https://golang.org/cl/5367043