FindBoost: Add explicit Boost_ARCHITECTURE option

Brad Kingrequested to merge
brad.king/cmake:FindBoost-explicit-arch-tag into master

Boost 1.66 and above built with --layout=versioned add an architecture tag to the library file names. We already try to compute this tag automatically when CMAKE_CXX_COMPILER_ARCHITECTURE_ID is available, but that is currently not computed everywhere. Add an explicit Boost_ARCHITECTURE option that a user can set to specify the architecture tag.

Issue: #17701

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