[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$'

-- go.mod --
module my.pkg

go 1.16
-- main/main.go --
package main

import "my.pkg"

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

var Text = "unset"