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://golang.org/cl/
7624043