Documentation/dev-tools: Use gcc version number instead svn revision number

svn commit 231296 matches commit d29e939c63b71 ("Add fuzzing coverage
support") in the gcc git. The change is part of gcc 6.1.0.

Replace the svn commit number with a gcc version which everyone can
easily compare.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>