target/hppa: Use explicit big-endian LD/ST API

Missing review: #2 and #3

Since v1:
- Rebased on commit 83202ca7227 (hppa-updates-qemu-v11-pull-request)

PA-RISC is big-endian. Use the explicit 'big'
endianness instead of the 'native' one.
Forbid further uses of legacy APIs.

tag: https://gitlab.com/philmd/qemu/-/tags/endian_hppa-v1
CI: https://gitlab.com/philmd/qemu/-/pipelines/2231223066