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