cmd/objdump: fix dissasembly of Plan 9 object files

Ignore symbols that aren't text, data, or bss since they cause
problems when dissassembling instructions with small immediate
values.

Before:
        build.go:142    0x10ee  83ec50      SUBL $text/template/parse.autotmp_1293(SB), SP

After:
        build.go:142    0x10ee  83ec50      SUBL $0x50, SP

Fixes #7947.

LGTM=rsc
R=rsc, 0intro
CC=golang-codereviews
https://golang.org/cl/93520045

src/cmd/objdump/objdump_test.go

@@ -155,10 +155,6 @@ var armNeed = []string{
 // can handle that one.
 
 func TestDisasm(t *testing.T) {
-	if runtime.GOOS == "plan9" {
-		t.Skip("skipping test; see http://golang.org/issue/7947")
-	}
-
 	tmp, exe := buildObjdump(t)
 	defer os.RemoveAll(tmp)