llext: fix list_symbols shell command

The shell command 'llext list_symbols' currently iterates over the
'sym_tab' table. This is valid only inside 'llext_load' and is freed
before returning, so it will always be empty in shell context.

Fix it to display 'exp_tab', which is the one used at runtime by LLEXT
to provide symbols to the main application.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>