Fix CPU POST test failure

The CPU POST test code (run from cpu_post_exec_31()) doesn't follow the
ABI carefully, at least the CR3, CR4, and CR5 fields of CR are clobbered
by it. The gcc-4.2 with its more aggressive optimization exposes this fact.
This patch just saves the CR value before running the test code, so allowing
it to do anything it wants with CR.

Signed-off-by: Dmitry Rakhchev <rda@emcraft.com>
Acked-by: Yuri Tikhonov <yur@emcraft.com>
--