ld: fix handling of "". names in #pragma dynimport/dynexport
ld: fix handling of "". names in #pragma dynimport/dynexport

Fixes issue 728.

R=r
CC=golang-dev
http://codereview.appspot.com/1706053
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go