x86: linkage - get rid of _X86 macros

Impact: cleanup

There was an attempt to bring build-time checking for
missed ENTRY_X86/END_X86 and KPROBE... pairs. Using
them will add messy in code. Get just rid of them.
This commit could be easily restored if the need appear
in future.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>