kernel: wipe TLS when dropping to user mode

For threads that run in supervisor mode for some time before
synchronously dropping to user mode, re-initialize the TLS
area to prevent leakage of potentially sensitive information.

We did this already for CONFIG_THREAD_USERSPACE_LOCAL_DATA
but not the new CONFIG_THREAD_LOCAL_STORAGE.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>