gcc: add a BR2_GCC_ENABLE_GRAPHITE option

The gcc graphite optimisations such as loop-interchange, blocking
and loop-flattening, also known as graphite are an optional feature of
gcc that is very well supported since about gcc version 4.5.

This patch adds support for graphite for the toolchain as an optional
flag for versions 4.8 onwards as an optional flag, that is disabled by
default.

Signed-off-by: Steve Thomas <scjthm@live.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>