Fix linker error if go_library with cgo references unresolved symbol (#3174)

Before this commit, if a go_library referenced an unresolved C symbol,
the build would fail with an error about an undefined symbol since
compilepkg.go tries to link all object files into an executable to be
consumed by cgo's -dynimport argument.

Since this library is never used for any purpose other than having its
symbols parsed by the cgo command, retry with additional linker flags
intended to make the linker ignore unresolved symbols. If they remain
missing in the real link step, they will still be reported as unresolved
there.

Fixes https://github.com/golang/go/issues/25832 for rules_go.