kbuild: fix SPL link bug when USE_PRIVATE_LIBGCC is "yes"

Commit 6825a95 (kbuild: use Linux Kernel build scripts)
changed the behavior of linkage when USE_PRIAVATE_LIBGCC
is defined as "yes".
(It dropped arch/arm/lib/eabi_compat.o from the
target library.)

Affected boards are all Tegra boards.

This commit gets back the same behavior as before Kbuild series.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Warren <twarren@nvidia.com>
Cc: Tom Rini <trini@ti.com>

spl/Makefile

@@ -133,7 +133,8 @@ libs-y := $(patsubst %/, %/built-in.o, $(libs-y))
 
 # Add GCC lib
 ifeq ("$(USE_PRIVATE_LIBGCC)", "yes")
-PLATFORM_LIBS := $(SPLTREE)/arch/$(ARCH)/lib/lib.a
+PLATFORM_LIBGCC = $(SPLTREE)/arch/$(ARCH)/lib/lib.a
+PLATFORM_LIBS := $(filter-out %/lib.a, $(filter-out -lgcc, $(PLATFORM_LIBS))) $(PLATFORM_LIBGCC)
 endif
 
 u-boot-spl-init := $(head-y)