asan: fix standalone malloc implementation


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@172529 91177308-0d34-0410-b5e6-96231b3b80d8
[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
[asan] asan_allocator2: fix the memalign handling (clear the first word of allocated chunk in case there is an old memalign magic)

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@172603 91177308-0d34-0410-b5e6-96231b3b80d8