aarch64: Fix context refcount leak on SVC dispatch

[robertream]

synchronous_handler acquires a refcount but only releases it on the fault path, leaking one ref per syscall. Since syscall_handler takes its own refcount, we can safely release the initial refcount before syscall dispatch.

[francescolavra]

Nice find!
However, it would be better to not not acquire the refcount at all (in synchronous_handler()) when taking the syscall path, instead of acquiring it and then releasing it. Also, setting f[FRAME_FULL] = true is redundant in the syscall path, since syscall_handler() does that.
So I think the correct fix would be to move the following 2 lines in synchronous_handler() so they are just before fault_handler fh = ctx->fault_handler.

    f[FRAME_FULL] = true;
    context_reserve_refcount(ctx);

By the way, the same bug is also present in the riscv64 code, in trap_exception(), and the same fix should be applied there too.