vec: Fix bootstrap on i686-linux, 32-bit darwin and AIX

vec: Fix bootstrap on i686-linux, 32-bit darwin and AIX

As mentioned earlier, embedded_size is broken on vecs of long long, double
etc. on some platforms, which breaks bootstrap.
E.g. on i686-linux, the problem is mostly with older GCC versions being used
as stage1 compiler (GCC 4.8 to 7.* in particular), because alignas (long long)
makes U 64-bit aligned, while when long long m_vecdata[1]; is in vec, it is
only 32-bit aligned.  We've tried various ways and the following one seems
to work, use the old way (offsetof (vec, m_vecdata)) for non-class types
as well as standard layout class types, i.e. whenever offsetof is guaranteed
to work, and for others use the new day (in that case we don't run into
problems with long long or other scalar types and for the structure layout
there is just a struct with a given alignment.

2020-08-14  Jakub Jelinek  <jakub@redhat.com>
	    Jonathan Wakely  <jwakely@redhat.com>

	* system.h: Include type_traits.
	* vec.h (vec<T, A, vl_embed>::embedded_size): Use offsetof and asserts
	on vec_stdlayout, which is conditionally a vec (for standard layout T)
	and otherwise vec_embedded.

Co-Authored-By: Jonathan Wakely  <jwakely@redhat.com>