kernel: flush cache before mod_init

This could be necessary as redirecting instructions from D$ directly to I$ as it seems.
Related: https://github.com/SpinalHDL/VexRiscv/issues/137

artiq/firmware/ksupport/lib.rs

@@ -498,6 +498,9 @@ pub unsafe fn main() {
 
     ptr::write_bytes(__bss_start as *mut u8, 0, (_end - __bss_start) as usize);
 
+    board_misoc::cache::flush_cpu_dcache();
+    board_misoc::cache::flush_cpu_icache();
+
     (mem::transmute::<u32, fn()>(__modinit__))();
 
     if let Some(typeinfo) = typeinfo {