gcc: rename option for ARC gcc

The ARC gcc version is now based on gcc 6.x and no longer gcc 4.8.x,
which makes the option BR2_GCC_VERSION_4_8_ARC a bit irrelevant, as is
the prompt of this option.

This commit therefore renames this option to BR2_GCC_VERSION_ARC, and
adjust its prompt as well.

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