[BPF] explicit warning of not supporting dynamic stack allocation

Currently, BPF does not support dynamic static allocation.
For a program like below:
  extern void bar(int *);
  void foo(int n) {
    int a[n];
    bar(a);
  }

The current error message looks like:
  unimplemented operand
  UNREACHABLE executed at /.../llvm/lib/Target/BPF/BPFISelLowering.cpp:199!

Let us make error message explicit so it will be clear to the user
what is the problem. With this patch, the error message looks like:
  fatal error: error in backend: Unsupported dynamic stack allocation
  ...

Differential Revision: https://reviews.llvm.org/D74521