Unwind: avoid warning about unused typedef

Move the definition of Elf_Addr typedef to the only place it is used, to avoid:

```
llvm-project/libunwind/src/AddressSpace.hpp:501:28: warning: unused typedef 'Elf_Addr' [-Wunused-local-typedef]
```

when compiling for Android with _LIBUNWIND_ARM_EHABI defined and
_LIBUNWIND_SUPPORT_DWARF_UNWIND not defined.

Patch by Joel Klinghed!

llvm-svn: 372427