cmd/gc: add space to export data to match linker expectations

cmd/gc: add space to export data to match linker expectations

The linker split PKGDEF into (prefix, name, def) pairs,
and defines def to begin after a space following the identifier.
This is totally wrong for the following export data:

        func "".FunctionName()
        var SomethingCompletelyUnrelated int

The linker would parse
    name=`"".FunctionName()\n\tvar`
    def=`SomethingCompletelyUnrelated int`
since there is no space after FunctionName.

R=minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7068051