x86/head/64: Move the __head definition to <asm/init.h>

Move the __head section definition to a header to widen its use.

An upcoming patch will mark the code as __head in mem_encrypt_identity.c too.

Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/0583f57977be184689c373fe540cbd7d85ca2047.1697525407.git.houwenlong.hwl@antgroup.com