x86/efi/mixed: Move mixed mode startup code into libstub

The EFI mixed mode code has been decoupled from the legacy decompressor,
in order to be able to reuse it with generic EFI zboot images for x86.

Move the source file into the libstub source directory to facilitate
this.

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