[LibFuzzer] Introduce a portable WeakAlias implementation.

Windows doesn't really support weak aliases, but with some
linker magic we can get something that's pretty close on
Windows.  This introduces an interface to accessing weakly
aliased symbols that will work on any platform.  Linker
magic changes to come in a separate patch.

Patch by Marcos Pividori
Differential Revision: https://reviews.llvm.org/D27235

llvm-svn: 288530