x86, entry: Build thunk_*.o only with preemption

thunk_*.o contains the thunks for preemption. It is empty
on systems without CONFIG_PREEMPTION. At least on a LTO build
objtool complains that there is no symbol table for the empty
file.

Only build thunk_*.o with CONFIG_PREEMPTION.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
1 file changed