[ASan] Support the asan_loadN/asan_storeN functions in the DLL thunk library

Summary:
This is required for linking DLLs with large functions exceeding
san-instrumentation-with-call-threshold.  One such function is
vp9_fdct16x16_sse2 in libvpx.

Reviewers: timurrrr

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4758