sparc64: fix 32bit load sign extension

- change return type of ldl_* to uint32_t to prevent unwanted sign extension
  visible in sparc64 load alternate address space methods
- note this change makes ldl_* softmmu implementations match ldl_phys one
Signed-off-by: Igor V. Kovalenko <igor.v.kovalenko@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
