[asan] change the default mapping offset on x86_64 to 0x7fff8000. This gives roughly 5% speedup. Since this is an ABI change, bump the asan ABI version by renaming __asan_init to __asan_init_v1. llvm part, compiler-rt part will follow

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174957 91177308-0d34-0410-b5e6-96231b3b80d8