target/loongarch: Use explicit little-endian LD/ST API

LoongArch is little-endian. Use the explicit 'little'
endianness instead of the 'native' one.
Remove some target_ulong uses. Forbid further uses of
legacy APIs.

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