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

ssrl/go,h8liu/golang,ssrl/go,pombredanne/go-deleteme,tv42/old-go,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,h8liu/golang,apparentlymart/golang,h8liu/golang,tv42/old-go,gnanderson/go,ssrl/go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,ssrl/go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,h8liu/golang,dsriram/golang,dsriram/golang,h8liu/golang,dsriram/golang,tv42/old-go,h8liu/golang,gnanderson/go,ssrl/go,h8liu/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,ssrl/go,apparentlymart/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme