Update Documentation for the gcc v4.9 upgrade requirement.
Fixes: 5429ef62bcf3 ("compiler/gcc: Raise minimum GCC version for kernel builds to 4.8") Fixes: 6ec4476ac825 ("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>