These functions are used by gl[Get]TexImage, which imposes no
alignment restructions on the void *pixels parameter.
This fixes an unaligned access in GTK's "gtk:gdk / memorytexture" unit
test on SPARC, which causes the test to fail.
Fixes: 45ae4434b5 ("util: Use bitshift arithmetic to unpack pixels.")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23482>