Some minor cleanups of recently-added bool vector code.

Some minor cleanups of recently-added bool vector code.

* conf_post.h (assume): Always return void.  Use lint version
only if GCC and MSC versions don't apply.
* conf_post.h (assume):
* data.c (USC_MSC_POPCOUNT, count_trailing_zero_bits):
Depend on _MSC_VER, not __MSC_VER, for consistency with
the rest of Emacs.
* data.c (bool_vector_spare_mask, popcount_size_t_generic)
(popcount_size_t_msc, popcount_size_t_gcc, popcount_size_t)
(bool_vector_binop_driver, count_trailing_zero_bits)
(size_t_to_host_endian): Now static, not static inline;
the latter isn't needed with modern compilers and doesn't
work with older compilers anyway.