[dfsan] Don't clear shadow on dlopen(NULL, flags)

This ports msan https://reviews.llvm.org/D14795 to dfsan.
dfsan, like msan, clears shadow for globals in a newly opened DSO in
case the DSO occupies the address of a previously labeled/poisoned area.
The operation should not happen on the main executable.

In addition, for a DT_EXEC executable, l_addr is zero and will lead to a
null pointer dereference in ForEachMappedRegion.