[ASan/Win] Don't apply dllexport to __asan_init in the DLL thunk

This fixes '___asan_init_v4 already defined' errors when linking some of Chromium DLLs.
Looks like one of the DLL is using a .lib produced while linking another DLL and it exploded after r212699.
I'm trying to come up with a small testcase...

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@212815 91177308-0d34-0410-b5e6-96231b3b80d8