qemu_v8: build hafnium using OP-TEE's clang toolchain

Hafnium removed the LLVM toolchain from its prebuilts submodule.
Instead, use OP-TEE's clang toolchain ('make clang-toolchains') if it
exists, or the one already in PATH.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/qemu_v8.mk b/qemu_v8.mk
index c61629a..b104385 100644
--- a/qemu_v8.mk
+++ b/qemu_v8.mk
@@ -392,7 +392,7 @@
 # Hafnium
 ################################################################################
 
-HAFNIUM_EXPORTS = PATH=$(HAFNIUM_PATH)/prebuilts/linux-x64/clang/bin:$(HAFNIUM_PATH)/prebuilts/linux-x64/dtc:$(PATH)
+HAFNIUM_EXPORTS = PATH=$(TOOLCHAIN_ROOT)/clang-$(CLANG_BUILD_VER)/bin:$(PATH)
 
 .hafnium_checkout:
 	git -C $(HAFNIUM_PATH) submodule update --init