semihosting: User defined operation, Tcl command exec on host

Enabling a portion (0x100 - 0x107) of the user defined semihosting
operation number range (0x100 - 0x1FF) to be processed with the help of
the existing target event mechanism, to implement a general-purpose Tcl
interface for the target available on the host, via semihosting
interface.

Example usage:
- The user configures a Tcl command as a callback for one of the newly
	defined events (semihosting-user-cmd-0x10X) in the configuration
	file.
- The target can make a semihosting call with <opnum>, passing optional
	parameters for the call.

If there is no callback registered to the user defined operation number,
nothing happens.

Example usage: Configure RTT automatically with the exact, linked
control block location from target.

Signed-off-by: Zoltán Dudás <zedudi@gmail.com>
Change-Id: I10e1784b1fecd4e630d78df81cb44bf1aa2fc247
Reviewed-on: https://review.openocd.org/c/openocd/+/6748
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/target.h

@@ -294,6 +294,15 @@ enum target_event {
 	TARGET_EVENT_GDB_FLASH_WRITE_END,
 
 	TARGET_EVENT_TRACE_CONFIG,
+
+	TARGET_EVENT_SEMIHOSTING_USER_CMD_0x100 = 0x100, /* semihosting allows user cmds from 0x100 to 0x1ff */
+	TARGET_EVENT_SEMIHOSTING_USER_CMD_0x101 = 0x101,
+	TARGET_EVENT_SEMIHOSTING_USER_CMD_0x102 = 0x102,
+	TARGET_EVENT_SEMIHOSTING_USER_CMD_0x103 = 0x103,
+	TARGET_EVENT_SEMIHOSTING_USER_CMD_0x104 = 0x104,
+	TARGET_EVENT_SEMIHOSTING_USER_CMD_0x105 = 0x105,
+	TARGET_EVENT_SEMIHOSTING_USER_CMD_0x106 = 0x106,
+	TARGET_EVENT_SEMIHOSTING_USER_CMD_0x107 = 0x107,
 };
 
 struct target_event_action {