z_thread_mark_switched_*: use z_current_get() instead of k_current_get()

k_current_get() may rely on TLS which might not yet be initialized
when those tracing functions are called, resulting in a crash.

This is different from the main branch as in that case the implementation
was completely revamped and neither k_current_get() nor z_current_get()
are used anymore. This is a much simpler fix than a backport of that
code, similar to the implication in commit commit f07df42d ("kernel:
make k_current_get() work without syscall").

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>