tcg: Use uintptr_t in tcg_malloc implementation

tcg: Use uintptr_t in tcg_malloc implementation

Avoid ubsan failure with clang-20,
  tcg.h:715:19: runtime error: applying non-zero offset 64 to null pointer
by not using pointers.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>