Add -r option to 6l/8l/5l.

Add -r option to 6l/8l/5l.

This permits more flexibility with cgo and swig in cases where
the program is run on a machine other than the one on which it
is built.  Rather than storing the absolute path to the shared
library in the DT_NEEDED entry, we can store just the name,
and let the dynamic linker find it using DT_RUNPATH or the
LD_LIBRARY_PATH environment variable.

R=rsc
CC=golang-dev
https://golang.org/cl/223068