sandbox: use correct type reading /proc/self/maps

Compiling arch/sandbox/cpu/os.c results in an error

../arch/sandbox/cpu/os.c: In function ‘os_find_text_base’:
../arch/sandbox/cpu/os.c:823:12: error: cast to pointer from
integer of different size [-Werror=int-to-pointer-cast]
  823 |     base = (void *)addr;
      |            ^
cc1: all warnings being treated as errors

The size of void* differs from that of unsigned long long on 32bit
systems.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>