Use CORE_ADDR as return type from x86_dr_low_get_addr

Use CORE_ADDR as return type from x86_dr_low_get_addr

On a Windows build locally, watchpoints started failing.  I tracked
this down to x86_dr_low_get_addr returning an 'unsigned long'... in
this particular build, this is a 32-bit type, but the inferior is a
64-bit program.

This patch fixes the problem by changing the return type.  No other
change is required, because this matches the function pointer in
struct x86_dr_low_type.