powerpc/nand spl: link libgcc

Recent GCC (4.4+) performs out-of-line epilogues in some cases, when
optimizing for size.  It causes a link error for _restgpr_30_x (and similar)
if libgcc is not linked.

It actually increases size with very small binaries, due to the fixed size
of the out-of-line code, and not having any functions that actually need to
restore more than 2 or 3 registers.  But I don't see a way to turn it off,
other than asking GCC to optimize for speed -- which may also increase
size for some boards.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Kim Phillips <kim.phillips@freescale.com>
Acked-by: Wolfgang Denk <wd@denx.de>