rcar/product: put back libc stub support for rcar

arch_libc.c was removed by commit 5616115f, because commit d35bad05 that create
cmake files switch from arch_libc.c to newlibc usage.
But newlibc is currently not working on CortexA53 architecture as the MMU is
not initialized: so restore the arch_libc file for rcar configuration only.

Signed-off-by: Florian Sylvestre <fsylvestre@baylibre.com>