Commit 37e4c7bfc264 fixed arm, ppc, x86 (32bit),
while it made wrong definition of x86_64. Fix that.

Also, add commentary to raw fork() implementation.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>