Add support for shared library build

[makkarpov]

This PR adds support for building SPIRV-Reflect as a shared library.

Sure enough, this library is not a perfect candidate to be shared (i.e. stack-allocated structs without any kind of layout or size stability guarantees), but at least this PR makes library usable via FFI interface from languages like Java.

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[spencer-lunarg]

LGTM - @chaoticbob mentioned how we should add a blurb to the README.md to mention this is possible and which cmake options to set

[makkarpov]

Ok, I will add mentions in README.md that shared library build is possible.

Also, to be on the safe side, I will also perform DLL testing on Windows with actual application. It should work (GCC cross-compilation succeeded, all exports are present), but it's better to check.