Generate the SPIR-V shaders automatically at build time (#2200)

Add custom commands and targets to automatically assemble and validate
the SPIR-V shaders used by the test.

Automatic assembly depends on finding `python3`, `spirv-as` and
`spirv-val`. `SPIRV_TOOLS_DIR` can be defined by the user during
configuration to provide an override path. Default behaviour assumes
that the binaries exist in `PATH`.

---------

Signed-off-by: Ahmed Hesham <ahmed.hesham@arm.com>

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.5.1)
+cmake_minimum_required(VERSION 3.12.0)
 
 set( CONFORMANCE_SUFFIX "" )
 set(CLConform_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})