Add MIPS bits/floatn.h.

This patch adds a MIPS-specific bits/floatn.h header.  This header is
identical to the ldbl-128 version except for the comment at the top;
the purpose is to ensure that a 32-bit MIPS build installs a header
that is the same as in a 64-bit MIPS build and so properly shows
_Float128 support to be available for 64-bit compilations, on the
general principle of an installation for one multilib providing
headers also suitable for other multilibs.

Tested with build-many-glibcs.py.

	* sysdeps/mips/ieee754/bits/floatn.h: New file.