examples: fix building on arm64

examples: fix building on arm64

Commit f9886bc60f42 ("Added arm64 assembly for examples/api crt0") tried to
add arm64 support to the examples but crt0.S does not even build for
qemu_arm64_defconfig with CONFIG_API=y, CONFIG_EXAMPLES=y:

    examples/api/crt0.S: Assembler messages:
    examples/api/crt0.S:32: Error:
        expected a register at operand 1 -- `ldr ip,=search_hint'
    examples/api/crt0.S:33: Error:
        unexpected register type at operand 1 -- `str sp,[ip]'
    make[2]: *** [scripts/Makefile.build:292: examples/api/crt0.o] Error 1

Do not define _start twice.
Use valid register names.
Move syscall_ptr and search_hint to the data section to avoid an invalid
relocation.

Fixes: f9886bc60f42 ("Added arm64 assembly for examples/api crt0")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>