mac: Trigger a real SIGSYS on 32-bit x86 during tests

syscall(0) results in SIGSYS on x86_64, but not 32-bit x86. Choose a
high number as a nonexistent syscall number. As of 10.12.4, the highest
known system call number is 521.

Test: crashpad_util_test Signals.Cause*
Change-Id: I82dbd210f0c90fe933898ea0d360b431b10d090e
Reviewed-on: https://chromium-review.googlesource.com/489826
Reviewed-by: Robert Sesek <rsesek@chromium.org>

util/posix/signals_test.cc

@@ -180,7 +180,7 @@ void CauseSignal(int sig) {
     case SIGSYS: {
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"
-      int rv = syscall(0);
+      int rv = syscall(4095);
 #pragma clang diagnostic pop
       if (rv != 0) {
         PLOG(ERROR) << "syscall";