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/st/common/common.mk b/plat/st/common/common.mk
index 7f93961..f49112d 100644
--- a/plat/st/common/common.mk
+++ b/plat/st/common/common.mk
@@ -43,7 +43,7 @@
 STM32MP_UART_PROGRAMMER		?=	0
 STM32MP_USB_PROGRAMMER		?=	0
 
-$(eval DTC_V = $(shell $(DTC) -v | awk '{print $$NF}'))
+$(eval DTC_V = $(shell $($(ARCH)-dtc) -v | awk '{print $$NF}'))
 $(eval DTC_VERSION = $(shell printf "%d" $(shell echo ${DTC_V} | cut -d- -f1 | sed "s/\./0/g" | grep -o "[0-9]*")))
 DTC_CPPFLAGS			+=	${INCLUDES}
 DTC_FLAGS			+=	-Wno-unit_address_vs_reg