kbuild: move "checkgcc4" to PowerPC archprepare

"checkgcc4" is used only for PowerPC.
Move it to arch/powerpc/config.mk.

To make sure gcc is new enough before beginning build,
run "checkgcc4" during "archprepare".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

Makefile

@@ -1114,15 +1114,6 @@ SYSTEM_MAP = \
 System.map:	u-boot
 		@$(call SYSTEM_MAP,$<) > $@
 
-# GCC 3.x is reported to have problems generating the type of relocation
-# that U-Boot wants.
-# See http://lists.denx.de/pipermail/u-boot/2012-September/135156.html
-checkgcc4:
-	@if test $(call cc-version) -lt 0400; then \
-		echo -n '*** Your GCC is too old, please upgrade to GCC 4.x or newer'; \
-		false; \
-	fi
-
 checkdtc:
 	@if test $(call dtc-version) -lt 0104; then \
 		echo '*** Your dtc is too old, please upgrade to dtc 1.4 or newer'; \

arch/powerpc/config.mk

@@ -35,5 +35,14 @@ endif
 
 # Only test once
 ifneq ($(CONFIG_SPL_BUILD),y)
-ALL-y += checkgcc4
+archprepare: checkgcc4
+
+# GCC 3.x is reported to have problems generating the type of relocation
+# that U-Boot wants.
+# See http://lists.denx.de/pipermail/u-boot/2012-September/135156.html
+checkgcc4:
+	@if test $(call cc-version) -lt 0400; then \
+		echo -n '*** Your GCC is too old, please upgrade to GCC 4.x or newer'; \
+		false; \
+	fi
 endif