cmd/compile: fix dolinkobj flag in TestAssembly

Follow-up to CL 37270.

This considerably reduces the time to run the test.

Before:

real	0m7.638s
user	0m14.341s
sys	0m2.244s

After:

real	0m4.867s
user	0m7.107s
sys	0m1.842s

Change-Id: I8837a5da0979a1c365e1ce5874d81708249a4129
Reviewed-on: https://go-review.googlesource.com/37461
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <munday@ca.ibm.com>

src/cmd/compile/internal/gc/asm_test.go

@@ -122,7 +122,7 @@ func (ats *asmTests) compileToAsm(t *testing.T, dir string) string {
 	for _, i := range ats.imports {
 		out := filepath.Join(testDir, i+".a")
 
-		if s := ats.runGo(t, "build", "-o", out, "-gcflags=-dolinkobj", i); s != "" {
+		if s := ats.runGo(t, "build", "-o", out, "-gcflags=-dolinkobj=false", i); s != "" {
 			t.Fatalf("Stdout = %s\nWant empty", s)
 		}
 	}