arch: enable private libgcc for m68k

This patch fixes u-boot hanging on the first printf("%x", val).

Some toolchains built without multilib enabled may produce
u-boot freezing on first u64 shift operation, as in
lib/vsprintf.c number() function.
Using our private libgcc solves the issue.

Setting private libgcc enabled at architecture level to avoid
similar issues, it should not harm.

Signed-off-by: Angelo Durgehello <angelo@kernel-space.org>