llvm-isel-fuzzer: Weak function invoke the ire of PE/COFF

It's non-trivial to use weak symbols in a cross platform way (See
sanitizer_win_defs.h in compiler-rt), and doing it naively like we
have here causes some build failures:

  http://lab.llvm.org:8011/builders/clang-with-thin-lto-windows/builds/1260

Instead of going down the rabbit hole of emulating weak symbols for
this very trivial dummy fuzzer driver, we can just rely on the fact
that we know which hooks any given fuzz target implements and forward
declare a normal symbol.

llvm-svn: 312354