native,platform: clear the FPU context in uc_example

* This will fix the SIGSEGV sent by 4.19 kernel. It seems the cause is
  that a page fault is generated when restoring the FPU context with an
  fxrstor64 instruction. To work around, we just clear the FPU context
  in uc_example, so the kernel will not try to restore the FPU context.

am/src/native/platform.c

@@ -137,6 +137,7 @@ static void init_platform() {
 
   // save the context template
   save_example_context();
+  uc_example.uc_mcontext.fpregs = NULL; // clear the FPU context
   __am_get_intr_sigmask(&uc_example.uc_sigmask);
 
   // disable interrupts by default