[vm/ffi] Support alignment attributes in struct fields

[dcharkes]

We should consider supporting __attribute__((aligned (16))) in dart:ffi. (Cousin to supporting the attribute packed #38158.)

class A extends Struct {
  Pointer<Int> a;

  @AlignedTo(8)
  Int8()
  int b; // aligned to 8 both on 32 bit and 64 bit.
}

This would enable platform independent bitfields in structs: dart-lang/native#406. (Related issue #38954.)