Documentation: update for gcc 4.9 requirement

Update Documentation for the gcc v4.9 upgrade requirement.

Fixes: 5429ef62 ("compiler/gcc: Raise minimum GCC version for kernel builds to 4.8")
Fixes: 6ec4476a ("Raise gcc version requirement to 4.9")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>