Make compiler-rt/trunk/lib/asan compileable with g++.

Patch by Timur Iskhodzhanov (timurrrr@google.com)

The double-extern thing is
http://llvm.org/bugs/show_bug.cgi?id=11869

And the #include <string[s].h> are only needed on Mac (see comments in
the original code)
and also including them might make strchr/index conflict with the g++
system headers (which don't follow the man pages, ouch!)

llvm-svn: 149129