package/gcc: fix ARC failure to build in 2 phases.

When working with GCC initial at override source dir mode the
HOST_GCC_INITIAL_POST_PATCH_HOOKS is not called and compilation failes.
The solution is to use HOST_GCC_INITIAL_POST_RSYNC_HOOKS since this hook
is being called at override source dir mode.

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