x86/boot: Move the EFI mixed mode startup code back under arch/x86, into startup/

x86/boot: Move the EFI mixed mode startup code back under arch/x86, into startup/

Linus expressed a strong preference for arch-specific asm code (i.e.,
virtually all of it) to reside under arch/ rather than anywhere else.

So move the EFI mixed mode startup code back, and put it under
arch/x86/boot/startup/ where all shared x86 startup code is going to
live.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20250401133416.1436741-11-ardb+git@google.com