llext: rename symbol struct identifiers with SLID

Similar to commit aeec014c, this
renames the identifiers to have prefix of "__llext_sym_name_"
instead of having suffix of "_sym_name" in LLEXT symbol tables
for SLID-enabled applications. This is needed to avoid confusing
the gen_device_deps.py script as it searches for objects (e.g.
__device_dts_ord_*) by matching name stems without regard to
suffixes.

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