Fuzzing for LLVM-libc

Fuzzing tests are used to ensure quality and security of LLVM-libc implementations.

Each fuzzing test lives under the fuzzing directory in a subdirectory corresponding with the src layout.

Currently we use system libc for functions that have yet to be implemented, however as they are implemented the fuzzers will be changed to use our implementation to increase coverage for testing.

Fuzzers will be run on oss-fuzz and the check-libc target will ensure that they build correctly.