xtensa: make local symbols static

xtensa: make local symbols static

Symbols that are not exported should be declared as static.

Change-Id: Ieb627f7f896e4663b0d5b18c4ab1853b39d23d03
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7167
Reviewed-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Reviewed-by: Ian Thompson <ianst@cadence.com>
Tested-by: jenkins