riscv: make unsafe user copy routines use existing assembly routines

The current implementation is underperforming and in addition, it
triggers misaligned access traps on platforms which do not handle
misaligned accesses in hardware.

Use the existing assembly routines to solve both problems at once.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250602193918.868962-2-cleger@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>