Add aligned memory allocators

When working with storage (especially GInputStream or GOutputStream) it
is preferred to use page-aligned buffers so that the operating system
can do page-mapping tricks as the operation passes through the kernel.

Another use case is allocating memory used for vectorised operations,
which must be aligned to specific boundaries.

POSIX and Windows, as well as the C11 specification, provide this kind
of allocator functions, and GLib already makes use of it inside GSlice.
It would be convenient to have a public, portable wrapper that other
projects can use.

Fixes: #2574

docs/reference/glib/glib-sections.txt

@@ -1393,6 +1393,11 @@ g_alloca0
 g_newa
 g_newa0
 
+<SUBSECTION>
+g_aligned_alloc
+g_aligned_alloc0
+g_aligned_free
+
 <SUBSECTION>
 g_memmove
 g_memdup