target-xtensa: fix gdb register map construction

target-xtensa: fix gdb register map construction

Due to different gdb overlay organization between windowed/call0
configurations core import script doesn't always work correctly.
Simplify the script: always copy complete gdb register map from overlay,
count registers at core registerstion time. Update existing cores.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>