spirv: Fix building with SCons

spirv: Fix building with SCons

The SCons build broke with commit ba975140d3c9 because a SPIR-V
function is called from Mesa main. This adds a convenience library for
SPIR-V and adds it to everything that was including nir. It also adds
both nir and spirv to drivers/x11/SConscript.

Also add nir/spirv modules to osmesa and libgl-gdi targets. (Brian Paul)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105817
Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Brian Paul <brianp@vmware.com>