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

Hexagon is little-endian. Use the explicit 'little'
endianness instead of the 'native' one.
Forbid further uses of legacy APIs.

tag: https://gitlab.com/philmd/qemu/-/tags/endian_hexagon-v3
CI: https://gitlab.com/philmd/qemu/-/pipelines/2231223066