Move mips from ports to libc.

I've moved the MIPS port from ports to the main sysdeps hierarchy.
Beyond the README update, the move of the files was simply

git mv ports/sysdeps/mips sysdeps/mips
git mv ports/sysdeps/unix/mips sysdeps/unix/mips
git mv ports/sysdeps/unix/sysv/linux/mips sysdeps/unix/sysv/linux/mips

and in addition to the ChangeLog entries here, I put a note at the top
of ports/ChangeLog.mips similar to those in other files.

Tested that disassembly of installed shared libraries for mips is the
same before and after this patch (except for ld.so where paths in
assertions are involved, as for arm).

	* sysdeps/mips: Move directory from ports/sysdeps/mips.
	* sysdeps/unix/mips: Move directory from ports/sysdeps/unix/mips.
	* sysdeps/unix/sysv/linux/mips: Move directory from
	ports/sysdeps/unix/sysv/linux/mips.
	* README: Update listing for mips-*-linux-gnu and
	mips64-*-linux-gnu.

	* sysdeps/mips: Move directory to ../sysdeps/mips.
	* sysdeps/unix/mips: Move directory to ../sysdeps/unix/mips.
	* sysdeps/unix/sysv/linux/mips: Move directory to
	../sysdeps/unix/sysv/linux/mips.