uclibc: cleanup handling of ARC uClibc version

This commit makes the ARC uClibc version handling explicit by adding a
BR2_UCLIBC_VERSION_ARC_GIT option, rather than only relying on the
selected architecture. This is needed in preparation to the
introduction of uClibc-ng support, which also supports the ARC
architecture: so we will now have two uClibc versions capable of
handling ARC.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>