spirv-tools: build static library as well

[crueter]

Not sure why this was off. It's particularly useful for certain CI applications where SPIRV-Tools is better off being linked statically

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[GunniBusch]

I think as per brews policy, it is preferred to have shared libs. https://docs.brew.sh/Acceptable-Formulae#shared-vs-static-libraries. And if there are static libs required formulae might ship both. (ignore this if you actually meant to have shared AND static)

[crueter]

Yeah I want to have shared and static. Their handling is kind of goofed up right now, which might cause the build failure; I do have a pending PR there to fix it KhronosGroup/SPIRV-Tools#6481 but it stalled because I'm lazy and also haven't called for reviews yet. Will investigate more when I next get access to my ARM linux machine