Fix bootstrap failure with clang++.

This patch fixes a bootstrap failure when using clang as the host
compiler.  Default arguments for class template member functions
should be added in the declaration, not the definition.

From Jason:

> 8.3.6 says "Default arguments for a member function of a class template shall
> be specified on the initial declaration of the member function within the
> class template."

	PR bootstrap/54484
	* vec.h (vec_t::embedded_init): Move default argument value
	to function declaration.

From-SVN: r190951