Add user-defined callback on write() calls.

Add dfsan_set_write_callback(), which sets a callback to be invoked when
a write() call is invoked within DFSan instrumented code.

Patch by Sam Kerner!

Differential Revision: http://reviews.llvm.org/D3268

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@207131 91177308-0d34-0410-b5e6-96231b3b80d8