arm64: lse: remove -fcall-used-x0 flag

x0 is not callee-saved in the PCS. So there is no need to specify
-fcall-used-x0.

Clang doesn't currently support -fcall-used flags. This patch will help
building the kernel with clang.
Tested-by: NNick Desaulniers <ndesaulniers@google.com>
Acked-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NTri Vo <trong@android.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/lib/Makefile

@@ -12,7 +12,7 @@ lib-y		:= clear_user.o delay.o copy_from_user.o		\
 # when supported by the CPU. Result and argument registers are handled
 # correctly, based on the function prototype.
 lib-$(CONFIG_ARM64_LSE_ATOMICS) += atomic_ll_sc.o
-CFLAGS_atomic_ll_sc.o	:= -fcall-used-x0 -ffixed-x1 -ffixed-x2		\
+CFLAGS_atomic_ll_sc.o	:= -ffixed-x1 -ffixed-x2        		\
 		   -ffixed-x3 -ffixed-x4 -ffixed-x5 -ffixed-x6		\
 		   -ffixed-x7 -fcall-saved-x8 -fcall-saved-x9		\
 		   -fcall-saved-x10 -fcall-saved-x11 -fcall-saved-x12	\