fix(build): update clang target for aarch64

With the LLVM toolchain delivered by Arm [1], the target aarch64-elf
which is defaulting to aarch64-unknown-unknown-elf is now unknown.
Replace it with aarch64-unknown-none-elf.

[1]: https://github.com/ARM-software/LLVM-embedded-toolchain-for-Arm/releases/tag/release-19.1.5

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I5a0fad1ee29838ef2c3a1bc8ecfba05aacf0a6d6
diff --git a/Makefile b/Makefile
index 5682f93..4ee3f12 100644
--- a/Makefile
+++ b/Makefile
@@ -148,7 +148,7 @@
 		TF_CFLAGS_aarch64	:=	-target aarch64-arm-none-eabi
 	else
 		TF_CFLAGS_aarch32	=	$(target32-directive)
-		TF_CFLAGS_aarch64	:=	-target aarch64-elf
+		TF_CFLAGS_aarch64	:=	-target aarch64-unknown-none-elf
 	endif
 
 else ifeq ($($(ARCH)-cc-id),gnu-gcc)