[fuzzer,CMake] Add config name for fuzzer lit test

Add config name for fuzzer lit test, to make it easier to identify failures are with which config.

Before this change, same lit tests with different configs will share the same test name.
```
********************
Failed Tests (2):
  libFuzzer :: fuzzer-flags.test
  libFuzzer :: fuzzer-flags.test
```
Actually this is a failure of two lit tests(two configs of the same test).

With this change, the names will be different.
```
********************
Failed Tests (2):
  libFuzzer-i386-default-Linux ::fuzzer-flags.test
  libFuzzer-x86_64-default-Linux :: fuzzer-flags.test
```

Reviewed By: MaskRay, vitalybuka

Differential Revision: https://reviews.llvm.org/D158696