x86/boot/compressed: Rename efi_thunk_64.S to efi-mixed.S

In preparation for moving the mixed mode specific code out of head_64.S,
rename the existing file to clarify that it contains more than just the
mixed mode thunk.

While at it, clean up the Makefile rules that add it to the build.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-2-ardb@kernel.org