use correct macro definitions for clang-cl

clang-cl, an MSVC-compatible frontend built on top of clang, defined
_MSC_VER *and* supports __attribute__ syntax.  The ordering of the
checks in jemalloc_macros.h.in, however, do the wrong thing for
clang-cl, as we want the Windows-specific macro definitions for
clang-cl.  To support this use case, we reorder the checks so that
_MSC_VER is checked first (which includes clang-cl), and then
JEMALLOC_HAVE_ATTR) is checked.  No functionality change intended.