toolchain: esp32: support esp32s3 in 'espressif' toolchain

CROSS_COMPILE_TARGET is defined based on CONFIG_SOC selected. So far
'esp32s3' SoC was not taken into account, so building Zephyr with
'espressif' toolchain (the only one supported for now) is was not possible
out of the box.

Add 'CROSS_COMPILE_TARGET_xtensa_esp32s3' CMake variable, same as it is
done for other SoC specific variables. That way toolchain paths are
automatically figured out and building succeeds.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>