rename processor::halt_no_interrupts to cli_hlt, which
logically matches existing processor::sti_hlt.

Make use of arch::halt_no_interrupts in core/power.cc,
enclose x64-specific code in ifdefs, as well as putting
code on AArch64.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>