mesa/main: Add a 'spirv' flag to gl_shader_program_data

This will be used by the linker code to differentiate between programs
made out of SPIR-V or GLSL shaders.

This was rejected in the past, assuming that it was equivalent to
check for "shProg->_LinkedShaders[stage]->spirv_data != NULL". But:

  * At some points of the linking process it would be needed to check
    if _LinkerShaders[stage] is present, so the full check would be:

    "shProg->_LinkedShaders[stage] != NULL &&
     shProg->_LinkedShaders[stage]->spirv_data != NULL"

  * Sometimes you would like to do some specific to SPIR-V
    independently of the stage, or for any stage. For example, "link
    all the uniforms, for all stages". In that case checking for the
    flag would be equivalent to iterate all the _LinkedShaders and
    check if there is any spirv_data available.

The former makes readibility really worse. Both could be solved by
adding two helpers. But adding a flag seems really more simple and
readable.

v2: added justification for the flag on the commit message (Alejandro)

Signed-off-by: Eduardo Lima <elima@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>