gmacros: add G_ALIGNAS macro

Please let me know if you would prefer a wrapper over gcc compiler extensions and if I'm missing something in the MR; also if the MR and commit follows GLib contribution guidelines.

I see the below files would also need an update:

• https://gitlab.gnome.org/tapaswenipathak/glib/blob/master/glib/docs.c
• https://gitlab.gnome.org/tapaswenipathak/glib/blob/master/docs/reference/glib/glib-sections.txt

Other way is to not add stdalign.h and wrap _Alignof and _Alignas directly, page 258.

Closes #1055 (closed).