x86/boot/compressed: Move efi32_pe_entry() out of head_64.S

Move the implementation of efi32_pe_entry() into efi-mixed.S, which is a
more suitable location that only gets built if EFI mixed mode is
actually enabled.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-7-ardb@kernel.org