gallium: fix more potential strict aliasing issues

gallium: fix more potential strict aliasing issues

In particular, gcc man page warns that
union a_union {
   int i;
   double d;
};

int f() {
   double d = 3.0;
   return ((union a_union *) &d)->i;
}

"might" not be ok (why not?), even though it doesn't seem to generate
any warnings. Hence don't use this and do the extra step to actually use
assignment to get the values in/out of the union.
This changes parts of 3456f9149b3009fcfce80054759d05883d3c4ee5.