x86/boot: Move .head.text into its own output section

In order to be able to double check that vmlinux is emitted without
absolute symbol references in .head.text, it needs to be distinguishable
from the rest of .text in the ELF metadata.

So move .head.text into its own ELF section.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20241205112804.3416920-15-ardb+git@google.com