Declare _DYNAMIC and dl_phdr_info in asan_linux.cc on FreeBSD

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@205852 91177308-0d34-0410-b5e6-96231b3b80d8
Enable building of sanitizers on FreeBSD

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@205919 91177308-0d34-0410-b5e6-96231b3b80d8