plat/kvm/arm: Add alignament attribute to spinlock struct

The arm64 spinlocks need to be 8 bytes aligned, otherwise they will
cause problems when using them with a `ldaxr` instruction.
This commit adds an alignament attribute that solves this problem.

Signed-off-by: Răzvan Vîrtan <virtanrazvan@gmail.com>
Reviewed-by: Renê de Souza Pinto <Rene.deSouzaPinto@opensynergy.com>
Reviewed-by: Marc Rittinghaus <marc.rittinghaus@kit.edu>
Approved-by: Michalis Pappas <mpappas@fastmail.fm>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: unikraft#383

arch/arm/arm64/include/uk/asm/spinlock.h

@@ -36,7 +36,7 @@
 
 #include <uk/arch/atomic.h>
 
-struct __spinlock {
+struct __align(8) __spinlock {
 	volatile int lock;
 };