jimtcl: update to tag 0.79

jimtcl: update to tag 0.79

OpenOCD is stuck at jimtcl tag 0.77 that is 3 years old.
The latest tag 0.79 (2019-11-20) is already used by debian build,
which packs jim library separately, as shown in [1]. Today only
the build for architecture powerpcspe is still not updated to
latest package version.
I have been using jim 0.79 since the day of the release, without
any issue.

Switch jimtcl to latest tag 0.79

[1] https://packages.debian.org/sid/openocd

Change-Id: I3426e68c32f88ecde74d4278303925423db451e0
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5403
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>