gallium/util: add fast half float conversion functions

gallium/util: add fast half float conversion functions

This adds a fast half float conversion facility to Gallium.

Mesa already contains such a facility, but using a much worse algorithm.

This one is an implementation of
www.fox-toolkit.org/ftp/fasthalffloatconversion.pdf
and uses a branch-less algorithm with some lookup tables small enough
to fit in the L1 cache.

Ideally, Mesa should start using these functions too, but I'm not sure
how to arrange that with the current build system.

A new "u_gctors.cpp" is added that defines a global C++ constructor
allowing to initialize to conversion lookup tables at library init.