x86: be careful about tailcall breakage for sys_open[at] too

Came up through a quick grep for other cases similar to the ftruncate()
one in commit 0a489cb3.

Also, add a comment, so that people who read the code understand why we
do what looks like a no-op.

(Again, this won't actually matter to any sane user, since libc will
save and restore the register gcc stomps on, but it's still wrong to
stomp on it)

Signed-off-by: Linus Torvalds <torvalds@osdl.org>