[TargetMachine] Set dso_local if asan is detected

AddressSanitizer instrumentation does not set dso_local on non-thread-local
global variables in -fno-pic and it seems to rely on implied dso_local to work.
Add a hack until we have fixed AddressSanitizer to call setDSOLocal() as
appropriate.

Thanks to Vitaly Buka for reporting the issue and suggesting the way to detect asan.