target: add generic Xtensa LX support

target: add generic Xtensa LX support

Generic Xtensa LX support extends the original Espressif/Xtensa
patch-set to support arbitrary Xtensa configurations, as defined in
a core-specific .cfg file.  Not yet fully-featured.  Additional
functionality to be added:
- Xtensa NX support
- DAP/SWD support
- File-IO support
- Generic Xtensa multi-core support

Valgrind-clean, no new Clang analyzer warnings

Signed-off-by: Ian Thompson <ianst@cadence.com>
Change-Id: I08e7bf8fa57c25b5d0cb75a1aa7a2ac13a380c52
Reviewed-on: https://review.openocd.org/c/openocd/+/7055
Tested-by: jenkins
Reviewed-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>