gallium: fix reference counting functions to be strict-aliasing compliant

gallium: fix reference counting functions to be strict-aliasing compliant

Historically, parts of mesa code are not strict-aliasing safe, hence
-fno-strict-aliasing is needed to compile (this got forgotten for scons
builds for gallium, which indeed not only caused compiler warnings but also
unexplicable crashes in non-debug builds). However, we should try to eliminate
code not complying with strict-aliasing code at least for gallium.
Hence change pipe_reference functions to make them strict-aliasing compliant.
This adds a bit more complexity (especially for derived classes) but is the
right thing to do, and it does in fact fix a segfault.