spirv: Move SPIR-V building to Makefile.spirv.am and spirv/meson.build

spirv: Move SPIR-V building to Makefile.spirv.am and spirv/meson.build

Future changes will add generated files used only from
src/compiler/glsl.  These can't be built from Makefile.nir.am, and we
can't move all the rules from Makefile.nir.am to Makefile.spirv.am (and
it would be silly anyway).

v2: Do it for meson too.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (the meson bits)
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com> (the automake bits)