Added arm64 assembly for examples/api crt0

Added arm64 assembly for examples/api crt0

I've encountered a problem when compiling the 'examples/api' directory for ARM64 in U-boot. The problem lies in the assembly code in 'examples/api/crt0.S' where the current CONFIG_ARM code is only 32-bit. When targeting ARM64, a 64-bit version is necessary.

I have proposed a fix by including a 'CONFIG_ARM64' section in the assembly code as shown below.  These changes have been check via https://github.com/u-boot/u-boot/pull/538.

Feedback is welcome.

Signed-off-by: Kalen Brunham <kalen.brunham@intel.com>