[gccgo] skip 'gccgo does not support -ldflags -X'

go build -o linkx$GOEXE -ldflags -X=my.pkg.Text=linkXworked my.pkg/main
exec ./linkx$GOEXE
stderr '^linkXworked$'

-- my.pkg/main/main.go --
package main

import "my.pkg"

func main() {
	println(pkg.Text)
}
-- my.pkg/pkg.go --
package pkg

var Text = "unset"