x86_64: get rid of the use of KPROBE_ENTRY / KPROBE_END

Impact: clean up assembly macros and annotations - with some object impact

entry_64.S is the only user of KPROBE_ENTRY / KPROBE_END on
x86_64. This patch reorders entry_64.S and explicitly generates
a separate section for functions that need the protection. The
generated code before and after the patch is equal.

Implicitly changing sections in assembly files makes it more
difficult to follow why the assembler is doing certain things.
For example,

.p2align 5
KPROBE_ENTRY(...)

was not doing what you would expect. Other section changes
(__ex_table, .fixup, .init.rodata) are done explicitly already.
Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Acked-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>