arch: xtensa: Use the alternate _Level4Vector routine on ESP32

For some reason, the ESP32 HAL defines XCHAL_EXCM_LEVEL to 3.  This
enables a version of _Level4Vector that doesn't work on this hardware.

Without complete visibility if the version that should work be axed,
keep both in the tree, but build the working other version instead
if building for ESP32.

Jira: ZEP-2556
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>