kernel: make k_current_get() work without syscall

We cache the current thread ID in a thread-local variable
at thread entry, and have k_current_get() return that,
eliminating system call overhead for this API.

DL: changed _current to use z_current_get() as it is
    being used during boot where TLS is not available.

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