Apply asm redirections in wchar.h before first use

Similar to d0fa09a7, but for wchar.h.  Fixes [BZ #27087] by applying
all long double related asm redirections before using functions in
bits/wchar2.h.
Moves the function declarations from wcsmbs/bits/wchar2.h to a new file
wcsmbs/bits/wchar2-decl.h that will be included first in wcsmbs/wchar.h.

Tested with build-many-glibcs.py.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

(cherry picked from commit c7509d49)