[asan] change the default mapping offset on x86_64 to 0x7fff8000. This gives...

[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

llvm-svn: 174957