kernel: userspace: manipulate _thread_idx_map on per-byte basis

The sys_bitfield_(clear/set)_bit() work on pointer size element.
However, _thread_idx_map[] is a byte array. On little endian
systems, the bitops should work fine. However, on big endian
systems, changing the lower bits may actually be manipulating
memory outside the array when CONFIG_MAX_THREAD_BYTES is not
multiple of 4. So modify the code to perform bit ops on
a per-byte basis.

Fixes #72430

Signed-off-by: Daniel Leung <daniel.leung@intel.com>