semihosting: add custom user command handler

semihosting: add custom user command handler

Custom user syscalls can be handled with target events in the TCL scripts.
This patch gives another opportunity to handle custom syscalls in the c files.
Besides that some utility functions are also exported for the custom handlers.

Signed-off-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Change-Id: Ice13d527540a0de0b2a8abda912ae3dcff3834b7
Reviewed-on: https://review.openocd.org/c/openocd/+/6889
Tested-by: jenkins
Reviewed-by: Ian Thompson <ianst@cadence.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>