[libFuzzer] Use dynamic loading for External Functions on Windows.

Replace weak aliases with dynamic loading.
Weak aliases were generating some problems when linking for MT on Windows. For
MT, compiler-rt's libraries are statically linked to the main executable the
same than libFuzzer, so if we use weak aliases, we are providing two different
default implementations for the same weak function and the linker fails.

In this diff I re implement ExternalFunctions() using dynamic loading, so it
works in both cases (MD and MT). Also, dynamic loading is simpler, since we are
not defining any auxiliary external function, and we don't need to deal with
weak aliases.
This is equivalent to the implementation using dlsym(RTLD_DEFAULT, FnName) for
Posix.

Differential revision: https://reviews.llvm.org/D29751

llvm-svn: 294687