[kernel] system_get_log should take a void*

Since it's not just text that's being returned in the buffer, switch the
argument from a char* to a void*.

src/include/j6/tables/syscalls.inc

@@ -1,6 +1,6 @@
 SYSCALL(0x00, system_log,        const char *)
 SYSCALL(0x01, system_noop,       void)
-SYSCALL(0x02, system_get_log,    j6_handle_t, char *, size_t *)
+SYSCALL(0x02, system_get_log,    j6_handle_t, void *, size_t *)
 SYSCALL(0x03, system_bind_irq,   j6_handle_t, j6_handle_t, unsigned)
 SYSCALL(0x04, system_map_mmio,   j6_handle_t, j6_handle_t *, uintptr_t, size_t, uint32_t)
 

src/kernel/syscalls/system.cpp

@@ -33,7 +33,7 @@ system_noop()
 }
 
 j6_status_t
-system_get_log(j6_handle_t sys, char *buffer, size_t *size)
+system_get_log(j6_handle_t sys, void *buffer, size_t *size)
 {
 	if (!size || (*size && !buffer))
 		return j6_err_invalid_arg;