arm64: Remove duplicated symbols

arm64: Remove duplicated symbols

When build U-boot with clang with using commands:

  $ make HOSTCC=clang xenguest_arm64_defconfig
  $ make HOSTCC=clang CROSS_COMPILE=aarch64-linux-gnu- \
		CC="clang -target aarch64-linux-gnueabi" -j8

The compiler reports error:

  /tmp/start-acdf31.s:330:1: error: symbol '_start' is already defined
  _start:
  ^

Because the symbol '_start' has been defined twice, one is defined in
arch/arm/cpu/armv8/start.S, another is defined in the header
boot0-linux-kernel-header.h.

To fix building failure, this patch removes the symbol '_start' from
boot0-linux-kernel-header.h.

Signed-off-by: Leo Yan <leo.yan@linaro.org>