compiler: Move spirv into a module of its own

For historical reasons, nir and vtn were compiled together,
and a bunch of vtn specific targets were defined in
src/compiler/meson.build.

Now that we can, make src/compiler/spirv produce an internal
library that depends on NIR, and is used by the drivers/tools.
Also move the vtn specific targets into that directory's
meson.build.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23668>

src/intel/compiler/meson.build

@@ -177,7 +177,7 @@ if with_intel_clc
     include_directories : [inc_include, inc_src, inc_mapi, inc_mesa, inc_gallium, inc_intel],
     c_args : [pre_args, no_override_init_args],
     link_args : [ld_args_build_id],
-    dependencies : [idep_nir, idep_mesaclc, idep_mesautil, idep_intel_dev],
+    dependencies : [idep_nir, idep_vtn, idep_mesaclc, idep_mesautil, idep_intel_dev],
     native : true,
   )
 endif