spl: fix SPL build of private libgcc

This fixes the SPL build to link with the SPL version of libgcc if
USE_PRIVATE_LIBGCC is set to "yes".  Previously it was linking with
the libgcc from the normal u-boot build because it gets set in
PLATFORM_LIBS and passed down the to the SPL build.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>