Do built-in types have default constructors?

Simple Answer: Technically No.

Long Answer:

No. But!

The syntax you use to initialize them makes them look like they are being constructed by a default constructor or a default copy constructor.

int x0(5);     // Looks like a constructor. Behaves like one: x is initialized.
int x1{5};

int y0();      // Fail. Actually a function declaration.
// BUT
int y1{};      // So new syntax to allow for zero initialization
int z0 = int();// Looks like a constructor. Behaves like a constructor (0 init).
int z1 = int{};

int a0(b);     // Again.
int a1{b};

So technically there are no constructors for basic-POD types. But for all intents and purposes they act just like they have a copy constructor and default constructor (when initialized with the braces).

If it looks like a duck and quacks like a duck, then its very duck like.

A constructor is a member function (constructors are fully specified in clause 12 of the C++ Standard, which covers special member functions like constructors and destructors).

A member function can only be defined for a class type (C++03 9.3/1 says "Functions declared in the definition of a class, excluding those declared with a friend specifier, are called member functions of that class").

So non-class types (including fundamental types, array types, reference types, pointer types, and enum types) do not have constructors.

I don't have a copy of The C++ Programming Language to read the context of the quote that "Built-in types also have default constructors," but I would guess that Stroustrup is either using the term "constructor" in a loose, non-technical sense, or the meaning of the term or the way in which it is used in the Standard changed between when the book was published and when the language was standardized. I'd guess the former is far more likely than the latter.

As others have pointed out, the Standard contradicts TC++PL in a few instances, often related to terminology. Bjarne Stroustrup himself summarizes the situation well:

(...) However, [TC++PL] is not a reference manual or the standards text. If you need 100% precise and complete information you'll have to consult the text of the ISO C++ standard. (...)