[arch][arm64] Add a dsb to context switch on smp system.
[arch][arm64] Add a dsb to context switch on smp system.

Fixes a crash where a thread would get preempted and move to another
cpu before executing the dsb in arm64_mmu_unmap. This would leave the
old mapping in the tlb, since the cpu that executed the dsb did not
have any pending tlb maintenance operations.

Change-Id: I3753d7020f983672966c125d8010f5f4f7c46ed1

sndnvaps/lk,littlekernel/lk,minglun-tsai/lk,sndnvaps/lk,littlekernel/lk,ErikCorryGoogle/lk,minglun-tsai/lk,sndnvaps/lk,travisg/lk,travisg/lk,ErikCorryGoogle/lk,littlekernel/lk,minglun-tsai/lk,minglun-tsai/lk,travisg/lk,ErikCorryGoogle/lk,littlekernel/lk,hollanderic/lkstuff,sndnvaps/lk,ErikCorryGoogle/lk,sndnvaps/lk,travisg/lk,hollanderic/lkstuff,hollanderic/lkstuff,hollanderic/lkstuff