sys/targets: don't attempt to run unit tests on unsupported TestOS arch

$ go test -v -count=1 -run=TestCover/32 ./pkg/runtest/
=== RUN   TestCover
=== PAUSE TestCover
=== CONT  TestCover
=== RUN   TestCover/32
    run_test.go:121: skipping due to broken compiler:
        gcc: error: unrecognized command-line option ‘-m32’; did you mean ‘-m31’?
--- PASS: TestCover (0.21s)
    --- SKIP: TestCover/32 (0.00s)
PASS
ok      github.com/google/syzkaller/pkg/runtest 0.318s

Signed-off-by: Alexander Egorenkov <eaibmz@gmail.com>

sys/targets/targets.go

@@ -634,6 +634,7 @@ func init() {
 		}
 		host := List[goos][arch]
 		if host == nil {
+			target.BrokenCompiler = fmt.Sprintf("TestOS %v unsupported", target.PtrSize * 8)
 			continue
 		}
 		target.CCompiler = host.CCompiler