cmd/ld: record complete runtime-gdb.py path again
cmd/ld: record complete runtime-gdb.py path again

This code never got updated after the liblink shuffle.
Tested by hand that it works and respects GOROOT_FINAL.

The discussion in issue 6963 suggests that perhaps we should
just drop runtime-gdb.py entirely, but I am not convinced
that is true. It was in Go 1.2 and I don't see a reason not to
keep it in Go 1.3. The fact that binaries have not been emitting
the reference was just a missed detail in the liblink conversion,
not part of a grand plan.

Fixes issue 7506.
Fixes issue 6963.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews, iant, r
https://codereview.appspot.com/87870048

wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old