Makefile: Fix build with USE_PRIVATE_LIBGCC

If USE_PRIVATE_LIBGCC is set the yes building fails with a missing
libgcc.a As we use partial linking now it is libgcc.o now.

Signed-off-by: Matthias Weisser <weisserm@arcor.de>

Makefile

@@ -269,7 +269,7 @@ LIBBOARD := $(addprefix $(obj),$(LIBBOARD))
 # Add GCC lib
 ifdef USE_PRIVATE_LIBGCC
 ifeq ("$(USE_PRIVATE_LIBGCC)", "yes")
-PLATFORM_LIBGCC = -L $(OBJTREE)/arch/$(ARCH)/lib -lgcc
+PLATFORM_LIBGCC = $(OBJTREE)/arch/$(ARCH)/lib/libgcc.o
 else
 PLATFORM_LIBGCC = -L $(USE_PRIVATE_LIBGCC) -lgcc
 endif