glsl: create gl_program at the start of linking rather than the end

glsl: create gl_program at the start of linking rather than the end

This will allow us to directly store metadata we want to retain in
gl_program this metadata is currently stored in gl_linked_shader and
will be lost if relinking fails even though the program will remain
in use and is still valid according to the spec.

"If a program object that is active for any shader stage is re-linked
unsuccessfully, the link status will be set to FALSE, but any existing
executables and associated state will remain part of the current
rendering state until a subsequent call to UseProgram,
UseProgramStages, or BindProgramPipeline removes them from use."

This change will also help avoid the double handing that happens in
_mesa_copy_linked_program_data().

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>