arch/posix: Add libfuzzer support

Add support for LLVM's libfuzzer utility.  This works by building an
executable with a "LLVMFuzzerTestOneInput()" entry point (which is
external to Zephyr, running in the host process environment!), which
it drives out of its own main() routine.  The toolchain API is exposed
as just another sanitizer variant, which is clean.

Signed-off-by: Andy Ross <andyross@google.com>