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 37bb78cb8c)

ChangeLog

@@ -1,3 +1,9 @@
+2017-10-19  Joseph Myers  <joseph@codesourcery.com>
+
+	[BZ #22322]
+	* sysdeps/mips/bits/long-double.h: Move to ....
+	* sysdeps/mips/ieee754/bits/long-double.h: ... here.
+
 2017-10-17  Romain Naour  <romain.naour@gmail.com>  (tiny change)
 
 	[BZ #22296]

NEWS

@@ -49,6 +49,7 @@ The following bugs are resolved with this release:
   [22296] Let signbit use the builtin in C++ mode with gcc < 6.x
   [22320] glob: Fix one-byte overflow (CVE-2017-15670)
   [22321] sysconf: Fix missing definition of UIO_MAXIOV on Linux
+  [22322] libc: [mips64] wrong bits/long-double.h installed
   [22325] glibc: Memory leak in glob with GLOB_TILDE (CVE-2017-15671)
 
 Version 2.26