x86/asm/entry: Move the 'thunk' functions to arch/x86/entry/

x86/asm/entry: Move the 'thunk' functions to arch/x86/entry/

These are all calling x86 entry code functions, so move them close
to other entry code.

Change lib-y to obj-y: there's no real difference between the two
as we don't really drop any of them during the linking stage, and
obj-y is the more common approach for core kernel object code.

Cc: Borislav Petkov <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>