gallium/util: rewrite global constructor system for half floats (GCC/MSVC only!)

gallium/util: rewrite global constructor system for half floats (GCC/MSVC only!)

NOTE: this commit will cause Gallium to fail to build on any compiler
      except GCC, the Microsoft C compiler and compatible compilers that
      claim to be one of those.

This commit removes the u_gctors.cpp mechanism, in favor of using
compiler-specific syntax to add global constructors from C files.

This solves the problem of u_gctors.o not being pulled from static
libraries and avoids using C++.

However, it needs compiler-specific support for every compiler.

The Microsoft C compiler support has not been tested.