Shrink pseudovectors a bit

Shrink pseudovectors a bit

sizeof (struct Lisp_Marker) was 32 on x86, where 24 would do.
Problem noted by Stefan Monnier in:
https://lists.gnu.org/r/emacs-devel/2018-09/msg00165.html
* src/bignum.h (struct Lisp_Bignum):
* src/frame.h (struct frame):
* src/lisp.h (struct Lisp_Vector, struct Lisp_Bool_Vector)
(struct Lisp_Char_Table, struct Lisp_Hash_Table)
(struct Lisp_Marker, struct Lisp_Overlay)
(struct Lisp_Misc_Ptr, struct Lisp_User_Ptr)
(struct Lisp_Finalizer, struct Lisp_Float)
(struct Lisp_Module_Function):
* src/process.h (struct Lisp_Process):
* src/termhooks.h (struct terminal):
* src/thread.h (struct thread_state, struct Lisp_Mutex)
(struct Lisp_CondVar):
* src/window.c (struct save_window_data):
* src/window.h (struct window):
* src/xterm.h (struct scroll_bar):
* src/xwidget.h (struct xwidget, struct xwidget_view):
Add GCALIGNED_STRUCT attribute.
* src/lisp.h (GCALIGNED_UNION_MEMBER): Renamed from GCALIGNED_UNION.
All uses changed.
(GCALIGNED_STRUCT_MEMBER, GCALIGNED_STRUCT, GCALIGNED): New macros.
All uses of open-coded GCALIGNED changed to use GCALIGNED.
(union vectorlike_header): No longer GC-aligned.
(PSEUDOVECSIZE): Yield 0 for pseudovectors without Lisp
objects that place a member before where the first Lisp object
member would be.