Support secure intra-SM function pointers

[jovanbulck]

Currently, we only output a warning when dereferencing function pointers in an SM. Compiler should error on dereferencing an unprotected function pointer, and else generate the following pseudo-code:

    if ( fptr < sm_text_start || fptr >= sm_text_end )
    {
      /* target function outside SM */
      goto sm_exit // with function argument meta data according to sm_exit calling conventions
    }
    else
    {
       /* target function inside SM */
       (*fptr)();
    }

[jovanbulck]

We might also consider (inlined) utility functions in sm_support.h to simplify untrusted argument pointer checking. E.g.,

int SM_FUNC(reader) outside_sm(void *p)
{
    return ( (p < (void*) &__PS(reader)) || (p >= (void*) &__PE(reader)) ) &&
           ( (p < (void*) &__SS(reader)) || (p >= (void*) &__SE(reader)) );
}