[tsan] Disable sysroot flag on FreeBSD

FreeBSD does not install a number of Clang-provided headers for the
compiler in the base system due to incompatibilities between FreeBSD's     
and Clang's versions. As a workaround do not use --sysroot=. on FreeBSD    
until this is addressed.

llvm.org/pr26651
Differential Revision:	http://reviews.llvm.org/D17383

llvm-svn: 261229