x86: Fix vsyscall on gcc 4.5 with -Os

This fixes the -Os breaks with gcc 4.5 bug.  rdtsc_barrier needs to be
force inlined, otherwise user space will jump into kernel space and
kill init.

This also addresses http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44129


I believe.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
LKML-Reference: <20100618210859.GA10913@basil.fritz.box>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@kernel.org>