target/ppc: Remove POWERPC_EXCP_STCX

Always use the gen_conditional_store implementation that uses
atomic_cmpxchg.  Make sure and clear reserve_addr across most
interrupts crossing the cpu_loop.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>