Before the switch to liblink, the linkers accepted the -c flag
to print the call graph. This change restores the functionality.

This came in handy when I was trying to audit the use of SSE
instructions inside the Plan 9 note handler.

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/73990043