arm64: kernel: Don't rely on objcopy to make code under pi/ __init

arm64: kernel: Don't rely on objcopy to make code under pi/ __init

We will add some code under pi/ that contains global variables that
should not end up in __initdata, as they will not be writable via the
initial ID map. So only rely on objcopy for making the libfdt code
__init, and use explicit annotations for the rest.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-47-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>