Kernel/riscv64: Specify correct alignment for `FPUState` struct

The signal handling code (and possibly other code as well) expects this
struct to have an alignment of 16 bytes, as it pushes this struct on the
stack.

Kernel/Arch/riscv64/FPUState.h

@@ -13,7 +13,9 @@ VALIDATE_IS_RISCV64()
 
 namespace Kernel {
 
-struct FPUState {
+// This struct will get pushed on the stack by the signal handling code.
+// Therefore, it has to be aligned to a 16-byte boundary.
+struct [[gnu::aligned(16)]] FPUState {
     u64 f[32];
     u64 fcsr;
 };