Add utility functions to allocate aligned memory backed by
mem_fd objects. Add interface to Gallium for same allocation.
It will be used in later commits for external memory support
in Vulkan/OpenGL.
Reviewed-by: Dave Airlie <airlied@redhat.com>
Tested-by: Heinrich Fink <hfink@snap.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12345>
