Use Clang instead of GCC for ATF build

Other GCC tools are still being used for ATF, but the compiler itself is
not.

Bug: 124390919
Change-Id: I8d7a9694a608ddb1bb7832dbe887fd2434352c49
diff --git a/project/qemu-atf-inc.mk b/project/qemu-atf-inc.mk
index 16ff4e0..73adad6 100644
--- a/project/qemu-atf-inc.mk
+++ b/project/qemu-atf-inc.mk
@@ -43,6 +43,7 @@
 ATF_WITH_TRUSTY_GENERIC_SERVICES ?= false
 
 ATF_MAKE_ARGS := SPD=trusty
+ATF_MAKE_ARGS += CC=$(CLANG_BINDIR)/clang
 ATF_MAKE_ARGS += CROSS_COMPILE=$(ATF_TOOLCHAIN_PREFIX)
 ATF_MAKE_ARGS += PLAT=$(ATF_PLAT)
 ATF_MAKE_ARGS += DEBUG=$(ATF_DEBUG)