x86: add memory clobber in switch_to()

Segment registers are reloaded, so we should add a memory clobber. The
generated assembly code is identical in my tests, but this doesn't mean
it is necessarily true for all configurations/compilers.

x86_64 already has the memory clobber.

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>