accel/tcg: Fix cpu_ld*_code_mmu for user mode

accel/tcg: Fix cpu_ld*_code_mmu for user mode

These routines are buggy in multiple ways:
  - Use of target-endian loads, then a bswap that
    depends on the host endiannness.
  - A non-unwinding code load must set_helper_retaddr 1,
    which is magic within adjust_signal_pc.
  - cpu_ldq_code_mmu used MMU_DATA_LOAD

The bugs are hidden because all current uses of cpu_ld*_code_mmu
are from system mode.

Fixes: 2899062614a ("accel/tcg: Add cpu_ld*_code_mmu")
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>