x86/efi/mixed: Factor out and clean up long mode entry

Entering long mode involves setting the EFER_LME and CR4.PAE bits before
enabling paging by setting CR0.PG bit.

It also involves disabling interrupts, given that the firmware's 32-bit
IDT becomes invalid as soon as the CPU transitions into long mode.

Reloading the CR3 register is not necessary at boot time, given that the
EFI firmware as well as the kernel's EFI stub use a 1:1 mapping of the
32-bit addressable memory in the system.

Break out this code into a separate helper for clarity, and so that it
can be reused in a subsequent patch.

Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>