x86/boot: Move early_setup_gdt() back into head64.c

Move early_setup_gdt() out of the startup code that is callable from the
1:1 mapping - this is not needed, and instead, it is better to expose
the helper that does reside in __head directly.

This reduces the amount of code that needs special checks for 1:1
execution suitability. In particular, it avoids dealing with the GHCB
page (and its physical address) in startup code, which runs from the
1:1 mapping, making physical to virtual translations ambiguous.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Dionna Amalie Glaze <dionnaglaze@google.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kevin Loughlin <kevinloughlin@google.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: linux-efi@vger.kernel.org
Link: https://lore.kernel.org/r/20250504095230.2932860-26-ardb+git@google.com