misc: dot_writer fixup

misc: dot_writer fixup

In large configs the tooltip may be greater then the maximum line
size graphviz supports when parsing the dot file (typically 16k).
Adding '/' causes graphviz to break the string in multiple lines
while parsing and works around this limitation.

Change-Id: I16a0030127de4165080de97f5213309eed9fdeca
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19208
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>