[compiler-rt/test] Use FileCheck -allow-empty instead of count 0

[compiler-rt/test] Use FileCheck -allow-empty instead of count 0

Hope this fixes:

http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-android/builds/27977/steps/run%20lit%20tests%20%5Bi686%2Ffugu-userdebug%2FN2G48C%5D/logs/stdio

```
: 'RUN: at line 8';   UBSAN_OPTIONS=suppressions=/var/lib/buildbot/sanitizer-buildbot6/sanitizer-x86_64-linux-android/build/compiler_rt_build_android_i686/test/ubsan/Standalone-i386/TestCases/Misc/Output/nullability.c.tmp.supp  /var/lib/buildbot/sanitizer-buildbot6/sanitizer-x86_64-linux-android/build/compiler_rt_build_android_i686/test/ubsan/Standalone-i386/TestCases/Misc/Output/nullability.c.tmp 2>&1 | count 0
--
Exit Code: 1

Command Output (stderr):
--
Expected 0 lines, got 2.
```

Not sure what this would be printing though, a sanitizer initialization message?