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/arm/board/arm_fpga/platform.mk b/plat/arm/board/arm_fpga/platform.mk
index f44b37d..c9c248f 100644
--- a/plat/arm/board/arm_fpga/platform.mk
+++ b/plat/arm/board/arm_fpga/platform.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2021-2023, Arm Limited. All rights reserved.
+# Copyright (c) 2021-2024, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -129,6 +129,6 @@
 
 bl31.axf: bl31 dtbs ${BUILD_PLAT}/rom_trampoline.o ${BUILD_PLAT}/kernel_trampoline.o ${BUILD_PLAT}/build_axf.ld
 	$(ECHO) "  LD      $@"
-	$(Q)$(LD) -T ${BUILD_PLAT}/build_axf.ld -L ${BUILD_PLAT} --strip-debug -s -n -o ${BUILD_PLAT}/bl31.axf
+	$(Q)$($(ARCH)-ld) -T ${BUILD_PLAT}/build_axf.ld -L ${BUILD_PLAT} --strip-debug -s -n -o ${BUILD_PLAT}/bl31.axf
 
 all: bl31.axf