Install correct bits/long-double.h for MIPS64 (bug 22322).

Similar to bug 21987 for SPARC, MIPS64 wrongly installs the ldbl-128
version of bits/long-double.h, meaning incorrect results when using
headers installed from a 64-bit installation for a 32-bit build.  (I
haven't actually seen this cause build failures before its interaction
with bits/floatn.h did so - installed headers wrongly expecting
_Float128 to be available in a 32-bit configuration.)

This patch fixes the bug by moving the MIPS header to
sysdeps/mips/ieee754, which comes before sysdeps/ieee754/ldbl-128 in
the sysdeps directory ordering.  (bits/floatn.h will need a similar
fix - duplicating the ldbl-128 version for MIPS will suffice - for
headers from a 32-bit installation to be correct for 64-bit builds.)

Tested with build-many-glibcs.py (compilers build for
mips64-linux-gnu, where there was previously a libstdc++ build failure
as at
<https://sourceware.org/ml/libc-testresults/2017-q4/msg00130.html>).

	[BZ #22322]
	* sysdeps/mips/bits/long-double.h: Move to ....
	* sysdeps/mips/ieee754/bits/long-double.h: ... here.

(cherry picked from commit 37bb78cb)