fix: add inline to constexpr vars and extern C to _mm_pause for C++ module compat

[guptapratykshh]

Add inline to namespace-scope constexpr variable definitions in__any.hpp, __config.hpp, __query.hpp, __utility.hpp, __let.hpp,and __meta.hpp to prevent duplicate symbol errors when multiple C++ module
units include the same stdexec headers. Also add extern "C" to the_mm_pause declaration in stop_token.hpp to match the C linkage expected by MSVC's intrinsic headers in module builds

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[ericniebler]

/ok to test 66700f0