[libFuzzer] Export external functions on tests.

We need to export external functions so they are found when calling
GetProcAddress() on Windows. But we can't use `__declspec(dllexport)` because
we want the targets to be completely independent from the fuzz engines and don't
depend on other header files. Also, we don't want to include platform specific
code managed with conditional macros.
So, the solution is to add the exported symbols with linker flags in cmake.

Differential revision: https://reviews.llvm.org/D29752

llvm-svn: 294688