build: use new toolchain variables for tools

This change migrates the values of `CC`, `CPP`, `AS` and other toolchain
variables to the new `$(toolchain)-$(tool)` variables, which were
introduced by the toolchain refactor patch. These variables should be
equivalent to the values that they're replacing.

Change-Id: I644fe4ce82ef1894bed129ddb4b6ab94fb04985d
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/plat/renesas/rcar/platform.mk b/plat/renesas/rcar/platform.mk
index c95590d..5718478 100644
--- a/plat/renesas/rcar/platform.mk
+++ b/plat/renesas/rcar/platform.mk
@@ -371,7 +371,6 @@
 .PHONY: rcar_srecord
 rcar_srecord: $(BL2_ELF_SRC) $(BL31_ELF_SRC)
 	@echo "generating srec: ${SREC_PATH}/bl2.srec"
-	$(Q)$(OC) -O srec --srec-forceS3 ${BL2_ELF_SRC}  ${SREC_PATH}/bl2.srec
+	$(Q)$($(ARCH)-oc) -O srec --srec-forceS3 ${BL2_ELF_SRC}  ${SREC_PATH}/bl2.srec
 	@echo "generating srec: ${SREC_PATH}/bl31.srec"
-	$(Q)$(OC) -O srec --srec-forceS3 ${BL31_ELF_SRC} ${SREC_PATH}/bl31.srec
-
+	$(Q)$($(ARCH)-oc) -O srec --srec-forceS3 ${BL31_ELF_SRC} ${SREC_PATH}/bl31.srec