cmd/6l, cmd/8l: fix BSD builds

Before this CL, running

        cd misc/cgo/test
        go test -c
        readelf --dyn-syms test.test | grep cgoexp

turned up many UNDEF symbols corresponding to symbols actually
in the binary but marked only cgo_export_static. Only symbols
marked cgo_export_dynamic should be listed in this mode.
And if the symbol is going to be listed, it should be listed with its
actual address instead of UNDEF.

The Linux dynamic linker didn't care about the seemingly missing
symbols, but the BSD one did.

This CL eliminates the symbols from the dyn-syms table.

R=golang-dev
TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/7624043