__libdw_cu_addr_base: Cast offset to uintptr_t before returning as pointer.

__libdw_cu_addr_base: Cast offset to uintptr_t before returning as pointer.

Fixes a GCC compile issue on the elfutils-debian-i686 buildbot builder:

dwarf_getlocation_attr.c: In function ‘addr_valp’:
dwarf_getlocation_attr.c:62:12: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
     return (unsigned char *) offset;
            ^
cc1: all warnings being treated as errors

Signed-off-by: Mark Wielaard <mark@klomp.org>