arm64: simple memcpy/memset alternatives to be used during early boot

Let's provide our own z_early_memset() and z_early_memcpy() rather than
making our own .bss clearing function that risk missing out on updates
to the main version.

Also remove extra stuff already provided by kernel_internal.h.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>