These functions may be OK to leave the G_GNUC_MALLOC attribute on,
because the only valid pointers in the storage areas they return are,
themselves, new pointers.

However, it’s a lot easier to remove the attributes now than to try and
diagnose miscompilations in future. The performance impact of this is
likely to be unmeasurable. If there are performance problems caused by
this, then they can be profiled and fixed case-by-case in future,
bearing in mind the possibility for miscompilation if G_GNUC_MALLOC is
readded.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

GNOME/glib#1465