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