In rL279903, @hiraditya added a second declaration of
__throw_runtime_error to include/__locale. The original declaration
was in __locale too, but @mclow.lists moved it from there to
stdexcept.
In FreeBSD we compile most things with gcc's -Wredundant-decls, which
warns about such redundant redeclations, so I would like to remove the
one in stdexcept, as all the other callers of the function already
include __locale anyway.
While here, move the declaration in __locale to just above its first
invocation.