[asan] enable asan_allocator2 by default on Linux. Will enable it on other platforms and remove the old one later, after additional testing. The new allocator is much faster and uses less memory

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@172531 91177308-0d34-0410-b5e6-96231b3b80d8